Cushing was hurt in the second quarter of the Texans' 23-17 win over the New York Jets on a low block by guard Matt Slauson. Cushing walked off the field on his own, but didn't return.
Coach Gary Kubiak said Tuesday that Cushing has a torn anterior cruciate ligament and will need surgery in two to three weeks, after the swelling subsides. Cushing was leading Houston in tackles (38) before Monday night's game.
"A big blow for our team," Kubiak said. "We have to have guys pick it up around him and we've got to find a way to move forward. We're all just disappointed for him. It's a tough day for him."
The Texans (5-0) play Green Bay (2-3) in Houston on Sunday night.
Tim Dobbins moved into Cushing's spot in the lineup against the Jets, and finished with four tackles and a pass deflection. Kubiak says Dobbins is "option No. 1" to replace Cushing in the lineup.
Dobbins played his first four NFL seasons with San Diego, including one season under Wade Phillips, now the Texans' defensive coordinator. He was traded to Miami during the 2010 draft, released after one season and signed by the Texans in August 2011. He played in 15 games last season and four this year, mostly on special teams.
"He's a veteran player, he's played with Wade before," Kubiak said. "It's a lot to ask, but that's why he's here. It's an opportunity for him as a player and we'll have to rally around him and like I said, make up for Cush's loss as a group."
Another eventual option may be Darryl Sharpton, who was placed on the Physically Unable to Perform (PUP) list during training camp with a hip injury. By league rules, he can't be activated until Week 7. The Texans have a bye week after playing Baltimore on Oct. 21, and Kubiak said he'll have a chance to closely evaluate Sharpton then.
"If he was available to come off and help us today, he's still not ready," Kubiak said. "But we do have some more time on our hands. We are pushing him toward that date, here in a few weeks, to where we can get a good look at him and see. There's still some progress to be made."
No matter who steps in, Cushing will be hard to replace.
A first-round draft pick in 2009, Cushing was named the Defensive Rookie of the Year. He made 133 tackles that year, the second-most in the AFC behind Baltimore's Ray Lewis (134).
Cushing again led the Texans in tackles last season (114) and also intercepted two passes and forced two fumbles. He's only missed four games in his three seasons, all to serve a suspension for a positive test for a banned substance.
Cushing answered questions about it, and now ranks as one of the most respected players in the locker room.
"Those guys really look to him," Kubiak said. "He's an emotional leader, the way he plays, the way he works, the way he practices. Cush loves it all. That's the thing about him. Guys feed off of that."
Kubiak said Cushing "was in pretty good pain" on the field on Monday night, and seemed to realize the severity of the injury. The team talked about losing Cushing at halftime, then made just enough plays in the second half to survive a gritty effort by the Jets.
"I think there was some added incentive to finish that thing up for Cush," Kubiak said.
Defensive end J.J. Watt produced another dominant performance, making six tackles, including a sack. He also deflected three passes -- one of them leading to Brice McCain's interception that ended a Jets' scoring threat.
Arian Foster carried the offense and helped Houston win time of possession for the fifth time this season. Foster rushed 29 times for 152 yards, both season highs.
Justin Forsett had four carries for six yards in place of regular backup Ben Tate, who missed Monday night's game with a toe injury. Kubiak said Tate "was not close" to playing and will be evaluated again on Wednesday.
Aside from the injuries, Kubiak has growing concerns about Houston's kick-return and coverage teams. Trindon Holliday averaged only 11 yards on three kickoff returns, and the Texans were penalized three times on returns.
Most alarming, though, Houston gave up a 100-yard kickoff return to New York's Joe McKnight, a momentum-changing play after the Texans had built a 20-7 lead. Houston also comfortably led Miami in the season opener before Marcus Thigpen ran back a punt 72 yards for a touchdown.
"I'm very concerned special-teams wise," Kubiak said. "We didn't get a ball past the 20 (on kickoff returns) last night, and we've given up two touchdowns. We're near the bottom of the league in covering kicks. We've got to address it. It's going to cost us, at some point. We've been fortunate, but it's not good enough right now."