When evaluating the performances of the rookies, you need to take into account several different factors: positive or negative impact, the amount that they've played, and you have to ask yourself whether he helps the team overall. This year's class for the Colts has a lot of potential and a few of the performers have been exceeding expectations.
Jack Mewhort has started in all but 2 games this season and has been relatively good in those games. Mewhort has a +0.6 rating on Pro Football Focus and has allowed only one sack this season which is an impressive number for a rookie. Where he needs to improve is on his penalty intakes. He has 5 penalties this year (in 7 games) and that puts him in the top 10 for guards and centers. He needs to limit the amount of penalties he takes. It's not often that you get a rookie offensive linemen outside the 1st round that starts the season and starts most games for you. Mewhort has been a revelation as a guard and his name hasn't been mentioned much, which is a good thing for an offensive lineman.
Donte Moncrief had a quiet start to the season and was on the bench most of the time, but has picked it up the last few games. His role has continued to increase. His first good game came against the Jaguars, but that game was such a blowout that those catches came when the game was out of hand. Throughout the year, we've gotten glimpses of how good he could be. His breakout performance was against the Steelers with a 7 catch, 113 yard, 1 touchdown performance. He showed why he will be the future at receiver (with TY Hilton) for the Colts. Moncrief's role has been limited but whenever he gets a chance, he performs.
Jonathan Newsome was the 5th round pick and no one expected much from him this season, but he's found a way to have an impact. Newsome has played a little amount in each game this season but about half of his snaps have come in the last three games. This year, Newsome has three sacks and a forced fumble. His three sacks is one behind the team lead (Bjoern Werner) and he's only played in about a third of the snaps. He's exceeded all expectations and if his role continues to expand, he'll be a key player for the Colts on defense.
Zach Kerr has been a pleasant surprise. He went undrafted and had an almost immediate impact starting in the preseason. Kerr has received a consistent snap count all season and has played anywhere between 16 and 37 snaps each game. They are giving him chances to succeed. What's surprising about Kerr is that he is mobile and agile for a 335 pounder and has three sacks this season, which is one behind the team lead. You don't see guys playing the 1 technique getting sacks, but he seems to be the exception. Kerr has provided great depth and has proven to be a good rotational player.
There's also Jonotthan Harrison, who has started five games at center. Overall, he's played about 56% of the snaps and has been involved in the bottleneck at the center position. Harrison's impact overall has not been great, but he's had some very positive games and some very bad games as well. He still lacks the consistency that you need from your center. The Colts do trust him at center and he has been the guy for the last few games, even with Khaled Holmes and A.Q. Shipley healthy.
So, who is the best rookie of these guys? If looking at who has had the biggest impact and who has helped the team the most overall, then it seems like Jack Mewhort is the answer. Mewhort has played the most snaps out of any rookie and on top of that, is playing at a position that has been weak for a while. So, he's had the biggest impact but has helped the team out overall? Yes he has. According to Pro Football Focus, Mewhort has had four positive games this year and would have a very positive impact overall had he not had a bad game against the Giants. Mewhort gives the interior offensive line stability. He's only allowed one sack this year, which is very good for anyone, let alone a rookie. If he continues to improve, he should become one of the better guards in the league.