It's pretty clear Petras isn't starting because of injury. The offense wasn't any better in terms of YPA or success rate, so I don't think Padilla won the job via performance.
Kirk said Spencer was banged up and didn't practice Tuesday and Wednesday. They worked him in at the end of the week and Spencer said he felt OK. Then in warm-ups, the coaches noticed he had less zip on the ball and they asked him again how he felt, said he felt fine. Then they pulled him when it became apparent he was not ok. This is all from Kirk's mouth.
I really don't think Padilla won the position with that performance. It was a 6.25 YPA performance. With Padilla in the game the success rate* for Iowa was 31.4%. That means they were behind the sticks 68.6% of the time he was in the game.
The 6.25 YPA is good enough to put Padilla at 112th in the nation. Right behind Temple's qb, 5 places behind Petras, and last in the Big Ten.
That success rate is not great. Even vs. Purdue, the success rate was 37%. There were just lots of turnovers in that game, leading to a loss of scoring opportunities for the offense. The Wisconsin game was easily the worst offensive performance by Petras, at ~21%. Padilla did ok, but not great.
To back that up, let's take a game where everything went right, Maryland. 51 points, you'd think the offense did well. Success rate was 36.3%. Let's look at a good total offense, Virginia - 1st in FBS, in their worst game points wise. On their worst offensive day, they had a success rate of 43.8%. Iowa's *best* day was 36.3%. On Virginia's best day (a loss where they scored 49 points), they had a 55.4% success rate. That is how far Iowa is behind on offense, in general. 18% difference between the best in the country and one of the worst on offense.
I could go back and just do passing plays and figure out which qb executes better in the passing game. But my bet is that Padilla's passing performance wouldn't be much better than Petras's average day. And it would not take into account audibles to runs in pre-snap reads.
What all these success rates tell me though is that even with the mobility and quick releases and good decisions Padilla made, something else killed the drives, still, and my guess is that he didn't check out of bad plays at the LOS often enough. The offense was not any better with Padilla at qb, situation-wise. He's first string this week because Petras is hurt.
If Padilla does well vs. Minnesota in YPA and success rate, then I'll say he won the job. Right now the jury has to still be out, for me.
* Success rate is independent of field position or turnovers or defensive scores or special teams scores, or big explosive plays. It is situation dependent. A successful play is: 50% of the necessary yards to gain on 1st down, 70% of the yards on second down, and all the remaining yards on 3rd down. A TD is an automatic success on downs 1 - 3. A 1st down on downs 1 - 3 is an automatic success. You count the total number of successful plays and divide by the total number of successes as a percentage. That gets you the success rate of a game.
It is a very good indicator of good offenses. ~37% is a good success rate (top 50 offenses are about this good).
What's awesome is that you can turn it around and subtract the opponent success rate from 100% and get the defensive stop rate, which will also eliminate explosive plays from the defensive stats.
It's much better at telling you how good the offense and defense is overall than YPP or TOP or points, because it's an indicator of how good your offense or defense is in any situation. Moving the sticks or preventing them from moving is how the game is played.