"Watching Ryan [Mallett] in practice every day, I believe it's gotten better and better," said O'Brien, who told both quarterbacks of the news early Wednesday morning. "His huddle command, his knowledge of the offense, he's been in this system for four years ... I think he's made a lot of progress whereas he's never played in a game or hasn't had many reps in a game. We're going to give him his shot to play."
Fitzpatrick, who has 11 touchdowns and eight interceptions this season, signed with the Texans as a free agent after former QB Matt Schaub was traded. O'Brien made the change after a game in which Fitzpatrick had his second-lowest Total QBR of the season and completed a season-low 13 passes.
"I didn't like it," Fitzpatrick said of the change. "I don't really agree with it, but I respect the hell out of Coach O'Brien. It's a hard thing, especially with me at this point in my career, to have to sit and take a back seat. But in talking with him and him redefining my role and what he expects out of me now, I'm going to play the good soldier and go into that role."
Fitzpatrick said he hasn't been benched during the season since his rookie year.
Meanwhile, Mallett relishes the idea of getting an opportunity after four NFL seasons during which he had yet to play a meaningful snap.
"It sure didn't seem like it, I'll tell you that," Mallett said when asked if he ever thought he would get this chance. "I'll tell you that. It's been four years. It didn't seem like it. ... I was starting to question if I'll ever get a shot. Well, now I've got a shot. We'll see what I do with it."
The Texans traded for Mallett in September, sending a draft pick to the New England Patriots. That pick is a conditional sixth-rounder on the condition that Mallett plays in 40 percent of the Texans' snaps this season, which he can accomplish by remaining the team's starter.
O'Brien said he intends to give Mallett a real chance at the position.
Mallett, a third-round draft pick in 2011, spent the first three seasons of his career as a backup to Tom Brady with the Patriots. O'Brien was the Patriots' offensive coordinator that season before he left to be Penn State's head coach in 2012.
O'Brien said that, upon Mallett's arrival in Houston this year, the coach saw a player who had "grown up" since he last saw him and that it was apparent that he had learned a lot from Brady.
Mallett sees growth in himself too.
"When he got me right after the draft, I was 22, 23. ... I was still a young kid," Mallett said. "Didn't really understand the league and how it worked and how much extra work needed to be put in. Now I do and I love it. It's fun. I love this game so much. It makes this easier."
"He has a much better knowledge of football and what it takes to be prepared," O'Brien said of Mallett. "Now it's time to put him out there and see what he can do."