HOUSTON -- Texans coach Gary Kubiak woke up Wednesday to the bad news he knew was probably inevitable.
Kubiak was hoping Schaub might still return this season after he was hurt in Houston's 37-9 win over Tampa Bay on Nov. 13. But Schaub met with several doctors and talked to Kubiak early Wednesday about the disappointing diagnosis.
"Well, that there's no way that any rest would allow him the opportunity to go play. It was just too risky," Kubiak said. "It's something he needs to get fixed for his future, so that's what we got to go do. We've got to go do what's best for him."
Matt Leinart will start for Houston (7-3) at Jacksonville (3-7) on Sunday. Rookie T.J. Yates, a fifth-round pick, will be the backup, and Clemens will take the No. 3 spot on the depth chart.
Schaub was averaging only 29 pass attempts per game, but had completed 61 percent of his throws this season. His 96.8 rating ranks sixth, and his 15 touchdown passes rank ninth.
The Texans haven't needed Schaub to put up big numbers like he did last year, because running backs Arian Foster and Ben Tate have both played well and helped Houston rank second in rushing (158 yards per game).
"It's not his fault we weren't throwing it that much," Kubiak said, "but when we threw it, he was throwing it as good as anybody and he made probably as many big plays as anybody in the league.
"Everything he was doing," Kubiak said, "he was a better play this year than he'd been for me the previous four years. I think Matt's going to have a great career."
Now, it's up to Leinart to help Houston earn its first playoff berth in franchise history. The Texans have a two-game lead in the AFC South with six to play.
Kubiak said Schaub will be back at practice on Thursday to assist in getting the other quarterbacks ready, and Leinart says Schaub will be on the sideline Sunday to help him.
"I'm going to pick up where he (Schaub) left off and get us into good situations and do what he was doing," Leinart said. "I know he's going to be there every day supporting me. I'm sure I'll turn to him for a bunch of things.
"We all know he'll do everything to get back as soon as he can," Leinart said. "But he's going to be here, helping big-time."
Clemens had an edge because he was in training camp with Washington and coach Mike Shanahan, Kubiak's former mentor. The Redskins' offensive coordinator is Kyle Shanahan, Mike's son, who was Kubiak's offensive coordinator with the Texans from 2008-09.
Since he was cut by the Redskins on Sept. 3, Clemens has mostly been working out in his home state of Washington, waiting for an opportunity to re-enter the league.
"The offense, there's some carryover," said Clemens. "It's been as easy a transition as it could be in one day. I've only been official for a few hours."
Johnson has been out since hurting his right hamstring in the Texans' 17-10 win over Pittsburgh on Oct. 2, an injury that required minor surgery to repair. Manning has made a quicker-than-expected recovery after breaking his left fibula in Houston's 41-7 win over Tennessee on Oct. 23.
Kubiak expects Johnson and Manning to both play in Jacksonville.
Schaub is the 10th Texans player to go on injured reserve this season, joining a group that includes outside linebacker Mario Williams (torn chest muscle) and linebacker Darryl Sharpton (dislocated shoulder).
Schaub's durability was questioned after he missed games in the 2007 and '08 seasons with various injuries. But the quarterback started all 16 games for Houston in 2009 and 2010.
Clemens was a second-round pick by the New York Jets in the 2006 draft and backed up Chad Pennington. Clemens started only nine games for the Jets, eight in the 2007 season when the team finished 4-12.
He was eventually demoted to No. 3 on the depth chart after Rex Ryan became the coach, and the team drafted Mark Sanchez in 2009. Clemens became a free agent after the 2010 season, signed with Washington before training camp began.