There are two sayings that come to mind about this situation.
"The most popular guy on a losing team is the backup quarterback"
"Be careful what you wish for"
During the first eight games of the season, there was a clamoring of sorts for Larry Fedora to hand the offense over to Williams. Some fans went as far as to advocate Williams become the full-time starter over the fifth year senior in his third season at that position. Fedora didn't see it that way and with good reason. It wasn't necessarily a clear cut choice other than keeping Renner as the starter because he did enjoy an edge in terms of experience and passing ability.
That being said, losing Renner is not the end of the season as it would be for other schools where the starter is well ahead of his backup. The splitting of snaps between Renner and Williams was smart because it allowed the Tar Heels to utilize the strengths of both players, forced the opposing defense to adjust and was somewhat effective in getting the offense moving. There is some complaint here about whether the constant switching of quarterbacks from one play to the next was a good idea. In other instances pulling a quarterback who had momentum on a drive to bring the other one in cold off the bench as Fedora did against Miami didn't end well for the Tar Heels.
All of that is gone now and this team will ride or die with Williams under center. While there are plenty of factors that point to this working out well for UNC, there is also a lot of uncertainty, some of which we might be able to decipher with a look at the stats.
The Passing Game
It will come as no shock that Renner's passing numbers are going to be better and there will be instances where his ability to make certain throws in crucial situations will be missed. One caveat to all of this is the fact Williams' passing has been limited to some degree because that wasn't his greatest strength. So the sample size here is smallish making it difficult to draw conclusions with the exception of one stat, interceptions. Williams has thrown three picks in only 66 pass attempts. That is a an interception every 22 attempts. Renner on the other hand, threw an interception every 46 attempts.
This is not a small issue. Williams is going to be asked to pass the football more often. How will an increased number of passing attempts impact his overall passing numbers? If Williams is prone to interceptions this often it could be problematic though it should be noted that two of the interceptions came against Virginia Tech in the sophomore's first career start. The Hokies boast a highly touted secondary so perhaps the numbers are not as dire as they might seem. The bottom line here is Williams needs to make good decisions when passing and be more accurate than he has been at times. The 60% completion rate is decent and if he can keep it roughly in that range and the interceptions in check, the passing game should be fine.
Related to that is a question of how much downfield passing Williams will do? Out of his forty completions, Williams has fifteen of 15+ yards and four over 25 yards. Renner had 46 completions beyond 15 yards and 14 of the 25+ yard variety out of 151 completed passes. That means Williams has a slightly higher percentage of his completions downfield than even Renner did. There are a lot factors that go into this, not the least of which is Williams as a run threat changes how the defense plays. Renner, who was more limited on his feet and played behind an average offensive line had less time to throw. Whether by play call or quarterback decision, Renner opted for shorter throws. It should also be noted that 11 of Williams 14 passes over 15 yards came with UNC trailing in the game.
The best conclusion we can draw here is that the passing game, accounting for Williams being less accurate and possibly making a few more ill-advise decisions, will regress some. That could swing the other direction is Williams' mobility, which opponents must honor, impact the pass coverage. A run threat quarterback usually means one fewer player in pass coverage and blitzing is not nearly as easy since Williams can escape. Whether that tilts the scales to make up for any deficiencies Williams might have in passing the football remains to be seen.
The Running Game
In addition to Williams' mobility changing how defenses play it also produces rushing yards. This was an area Renner had a negative impact, especially given his penchant for taking bad sacks. Williams is less likely to do that and in some instances he will use his legs to get yards the somewhat ineffective rushing attach sorely needs. As it stands, Williams is UNC's third leading rusher in terms of total yards, is second in yards per game and third in yards per carry. Williams' 8.3 rushing attempts per game is second only to A.J. Blue. His effectiveness as a runner, especially in a spread offense, is a huge plus. However the greater value might be in avoiding being sacked which was a surprising weakness of Renner's.
Fedora has already said that because of the attrition at quarterback, they probably can't afford to intentionally run Williams as often as they did before. That doesn't mean he won't get his share of called runs and with the way the Tar Heel offensive line has played, scrambles for yardage are certainly in the mix. Williams' ability to negate an average offensive line could be a nice bonus as the next four games unfold with him at the helm.
One huge factor is how opposing defenses will plan and respond knowing ahead of time they are only facing Williams. To this point, UNC has enjoyed the luxury of a two quarterback system which requires opposing defense to prepare for a myriad of looks from two quarterbacks which different strengths. Now, it is only Williams and defense can prepare for how the offense operates under him. That will have some impact but how much is speculation.
Another factor is how Williams responds to being "the guy" and the leeway that goes with it. To Fedora's credit he didn't operate the dual-quarterback system by yanking a player for making a poor decision on a given play. They generally followed a plan or changed out the position as the situation dictated. Williams has not safety net now, he also has the freedom to "make mistakes" which can make a player generally looser. There is also the element of getting into a rhythm which Williams did when he was given an entire series to run rather than a quick substitution. With some players, being thrust into the starting role and not worrying about the playing time is a huge lift. That could be the case here though Williams has been good even while sharing snaps with Renner.
It should also be noted that Williams isn't playing the position in a vacuum. So much of our analysis of quarterbacks often leans(wrongly) heavily on this notion that everything that happens on the field or with their own play is solely in their control. That is not the case. UNC still needs the rushing attack from the traditional tailbacks to improve. The receiving corps is starting to show some life beyond Eric Ebron with Quinshad Davis and Bug Howard playing much better. Those three receivers are big and can be a nightmare for opposing secondaries. And it would help tremendously if the offensive line could block plays without holding three or four times per game. On defensive side, things have to continue to trend as they have. Williams can be the second coming of Darian Durant and it won't matter if the defense goes back to giving up big plays on a regular basis.
For the most part, the loss of Renner and the move to Williams exclusively probably doesn't change the season math that much. The expectation remains that UNC will get wins over Virginia and Old Dominion then face a tough road game versus Pitt and Duke at home to close the season. Both those games are very losable and would doom the Heels to a 5-7 mark assuming UNC wins the other two. At the same time, the benefit of Williams having played effectively this season means this isn't a major drop off. The offensive players are clearly comfortable with Williams and at worst, he is getting four games of experience ahead of being the presumptive starter next season.
Stats source: CFB Stats