"Jimmy is a very talented guy," O'Brien said on a conference call with Bay Area reporters. "He's got a quick release. He's got very good instincts as to how to play the position. Accurate passer. I know how they felt about him in New England, and I know it's going to be a tough challenge for us.
"Jimmy plays the game in a very efficient way. He just does a good job of reacting and playing the position with good instincts and understanding of what the coach wants from the play and how to execute the play."
Garoppolo studied Brady for years before he joined the New England Patriots in 2014 as a second-round draft pick. O'Brien was on the Patriots' offensive staff from 2007 to 2011. Garoppolo was Brady's backup for 3 1/2 seasons before he was sent to the 49ers in a trade for a second-round draft pick.
Garoppolo had an impressive starting debut with the 49ers, throwing for 293 yards and leading a fourth-quarter comeback for a 15-14 victory over the Chicago Bears. Garoppolo will get his second start for the 49ers on Sunday against the Texans.
Garoppolo takes over as the 49ers starter after Brian Hoyer opened the first six games. C.J. Beathard started the next five before Garoppolo got his chance.
"I wouldn't say they changed a whole lot," O'Brien said. "I think Kyle (Shanahan) does a great job. He's one of the best play-callers in this league – and play-designers. And I think they do things to suit the skillsets of their quarterbacks and the players around him.
"They're building a foundation there and they're building it the right way. They are pro coaches, so they understand when a guy comes in after the halfway point, they got to get the guy ready to play and that's what they do."