Two serious NFL playoff contenders go to Plan B at quarterback Sunday. It is by necessity, not choice, but now the Houston Texans and Chicago Bears must try to demonstrate that they can continue to thrive with unproven backups forced into the lineup.

The Texans are atop the AFC South but have lost Matt Schaub to a season-ending foot injury, and will turn to Matt Leinart at quarterback for their game at Jacksonville. The Bears are in the thick of the NFC wild card chase but must play without Jay Cutler, who underwent surgery Wednesday for a fractured thumb on his throwing hand. Caleb Hanie takes over for their game at Oakland.

Both losses are significant: Schaub is the league’s sixth-rated passer and Cutler ranks 12th. The question to be answered starting Sunday is how much each team’s chances to reach the postseason and advance in the playoffs have been affected.

Some in the sport lament the quality of the league’s backup quarterbacks, maintaining that teams rarely have two capable, experienced passers on the roster in the salary cap era because of financial restraints. Whether that’s true or not, it’s clear that there is a high level of uncertainty whenever a starting quarterback gets hurt.

A team can go the way of the 2008 New England Patriots, who lost Tom Brady to a season-ending knee injury in the opening game but managed to go 11-5 with unheralded Matt Cassel at quarterback. Or it can go the way of this season’s Indianapolis Colts, who are winless with quarterback Peyton Manning sidelined as he works his way back from two neck surgeries.

“To lose your starting quarterback, it’s the most difficult thing to deal with,” said Brian Billick, the former Super Bowl-winning coach for the Baltimore Ravens who is now an analyst for Fox and the NFL Network. “Just look at the Indianapolis Colts and you can see that. Having said that, I think those two teams [the Texans and Bears] are pretty well positioned to deal with it and still get to the playoffs. . . . It may be in the playoffs where you see the effects of it.”

The Texans, with a 7-3 record, have a two-game lead in the AFC South as they attempt to reach the playoffs for the first time in the franchise’s 10-year history. No other team in the division is above the .500 mark, including the Colts.

With Schaub on the field, the Texans were a balanced team. They’re ranked second in the league in rushing, fifth in scoring offense and first in total defense.

“I think they’re challenged by it,” Texans Coach Gary Kubiak said of his players at a midweek news conference. “I think that’s the key. . . . Not a lot of people think they can continue to play at that level, [but] they believe they can and I believe they can.”

Leinart, the former Heisman Trophy winner at Southern California, was selected by the Arizona Cardinals with the 10th overall choice in the 2006 NFL draft and made a combined 16 starts in his first two seasons. But this is only his second regular season start since then. He lost the starting job in Arizona to Kurt Warner, who took the Cardinals to a Super Bowl in the 2008 season, and couldn’t reclaim it even after Warner retired following the 2009 season. The Cardinals released him before last season. This is his second season as Schaub’s backup in Houston.

With the Texans coming off their bye week, Leinart had an extra week to prepare for this start. Wide receiver Andre Johnson appears set to return to the lineup after missing six games with a hamstring injury. The offense can lean on running backs Arian Foster and Ben Tate. The defense, overseen by new coordinator Wade Phillips, has overcome the loss of linebacker Mario Williams to a season-ending torn pectoral muscle to continue to play extremely well. The Texans are on a four-game winning streak.

The Bears, too, are on a roll, with five victories in a row. They trail the unbeaten Green Bay Packers by 31 / 2 games in the NFC North but, at 7-3, lead the conference’s wild-card race. The Bears have expressed optimism that Cutler will play again this season but, for now, the job belongs to Hanie. The fourth-year pro has never started an NFL game but took over in last season’s NFC title game after Cutler suffered a knee injury, completing 13 of 20 passes for 153 yards. Hanie threw one touchdown pass and two interceptions in that loss to the Packers.

“It’s a great opportunity,” Hanie said at a midweek news conference. “You never want it to come at the expense of one of your good friends and close teammates and one of your best players. . . . But you have to take advantage of opportunities when they’re given to you. So that’s what I’m going to try to do.”

The Bears’ opponent Sunday, the Raiders, lost quarterback Jason Campbell to a shoulder injury earlier this season. But because Campbell was hurt before last month’s NFL trading deadline, the Raiders were able to obtain quarterback Carson Palmer from the Cincinnati Bengals.

It does not appear there is any such help on the way for the Bears after their potential reunion with their former starter, Kyle Orton, failed to materialize. Orton, released by the Denver Broncos, was awarded to the Kansas City Chiefs off waivers Wednesday.

Tailback Matt Forte is the league’s fourth-leading rusher and a major pass-receiving threat out of the backfield.

The Bears must hope a defense that ranks only 25th overall in the league (based on yards allowed) and 30th against the pass can do much of the heavy lifting now.

Bears Coach Lovie Smith expressed confidence that Hanie can do his part, saying at a news conference during the week: “He’ll do a good job at what we ask him to do, simple as that.”