"He took a big step forward in a lot of the small details of quarterback play," Pettine said Tuesday. "It's as simple as that for us, just to continue to get better. He did some good things but there's some things that can be improved upon.
"Each week presents a new challenge, and he has to be able to react to how a defense is playing him and be able to play within a game plan, the structure of it and stay tight to that."
Said Manziel: "It's a great opportunity for me. I was a little caught off-guard, just watching film this morning and Coach Pettine kind of came in and grabbed me. I was very excited."
McCown, who threw for 1,897 yards, 11 touchdowns and four interceptions in his seven starts this season, said Monday that he'd "do everything I can to help him to play the best ball he can to help us win football games."
However, not everyone involved in the decision wanted to commit to Manziel for the rest of the season because of how well McCown has played, a source told ESPN's Jeremy Fowler.
The Browns (2-8), in the midst of a five-game losing streak, have a bye this week before hosting Baltimore on Nov. 30.
After he wasn't ready to officially name Manziel the starter for the rest of the season on Monday, Pettine offered up praise Tuesday by saying Manziel had what was by far his best game in his nearly two seasons in Cleveland.
"I think the performance against Pittsburgh validated for us the progress that he's made, the improvement that he's shown and that he deserves this opportunity," Pettine said Tuesday. "Also, just Josh's circumstances being what they are, not being entirely healthy at least in the short term.
"It's something that we understand where our season is at this point and this is a kid we invested a lot in. This will give us an opportunity to see how far he's come and what he's capable of."
Manziel has started three games this season and has thrown for 933 yards, five touchdowns and two interceptions.
"I was just looking forward to coming out and trying to have a little bit of a spark," Manziel said of Sunday's performance. "And more than anything get some wins."
McCown, who went 1-6 as the starter, was brought in to mentor Manziel, and by all accounts, the 36-year-old has helped groom the talented and troubled quarterback.
"We'll move forward accordingly," McCown said Monday. "Just from a philosophical standpoint, if you pick somebody in the first round and they start to play and start to do some good things, I understand it if that's the decision."
Manziel, the 2012 Heisman Trophy winner, followed a rough rookie season by spending 70 days in a rehab facility for an unspecified condition. Manziel had been under league investigation for a recent domestic incident but on Tuesday, the NFL said it would not take any disciplinary action.
"I let him know that I'm not going to do anything that's going to be a distraction to this team or be an embarrassment to the organization," Manziel said.
"I'm going to come back and gear up for these last set of games that we have and come in fresh-minded, ready to go and excited to be back on the football field. So I don't think they're going to have to worry about me this week."
The Associated Press contributed to this report.