B+.
11 regular season/conference tournament losses were the most we've seen in a long time. That being said, a lot of the things that went wrong this year were not his fault.
First, he was hired extremely late because Hoiberg had to wait so long for the Bulls job to open up - so late that he had virtually no input on recruiting for 2015-16. While it's unlikely any freshmen he would have recruited would have contributed much this season, he missed out on the opportunity to recruit any potential immediately-eligible transfers that might have been able to contribute, especially transfers that would have been able to help McKay and Niang in the post and with rebounding.
Second, while he inherited a highly talented, veteran roster, that veteran roster was largely set in its ways. While Prohm coaches a similar style to Hoiberg, there are enough differences, and it wasn't until later in the season that he was able to figure out the balance between "status quo" with what the players were used to and "rocking the boat" to implement the differences.
Third, once again, while it was a talented, veteran roster, it was a flawed roster, and the flaws became highly apparent as he had to deal with injuries (Long), players going "mental" (McKay), players not doing what Prohm asked of them (Cooke), and players simply not ready to contribute outside of spot minutes (Carter, Babb, and Ashton) during the year. This led to having no depth throughout the year, at times going only 6 deep. Several posters in the past have argued that a lineup really only needs to go 7-8 deep. We found out this year that this is not the case. Perhaps if we had more depth and bigger bodies we might have been able to play more physical defense and be more physical on the boards because we would have more fouls to play with.
That being said, until last night, we didn't lose a game this year by more than 10 points, which means, in spite of all of the disadvantages Prohm had going against him, we were in every single game playing one of the nation's most difficult schedules. And even though we lost 11 games, the Committee still rewarded us with a 4-seed in the NCAA Tournament.
I think had a few more things gone our way this year this would have been an easy A.