HOUSTON — With two games left in regular season — against the Baltimore Ravens and Jacksonville Jaguars –the Houston Texans’ quarterback situation could not be any worse.

Case Keenum, signed off St. Louis’ practice squad on Monday, could be the starter against the Ravens.

Thad Lewis, who has been on the roster for three weeks, also could start, but he doesn’t know coach Bill O’Brien’s system as well as Keenum, who played for the Texans until he was waived on Aug. 31.

The Texans are in this predicament because veteran Ryan Fitzpatrick (fractured tibia) and rookie Tom Savage (hyperextended knee) went down in Sunday’s 17-10 loss at Indianapolis.

Fitzpatrick underwent surgery on Monday. He will join Ryan Mallett (torn pectoral muscle) on injured reserve. Fitzpatrick was injured when he was tripped at the end of a 2-yard run.

Savage, a fourth-round pick, suffered the knee injury when he was hit in the pocket. He limped off but returned for one more play, an interception. He is not expected to play the rest of the season.

Lewis was signed when Mallett suffered his season-ending injury. He has been the third quarterback behind Fitzpatrick and Savage.

Keenum, who originally signed with the Texans as an undrafted free agent in 2012, spent his rookie year on the practice squad. He started eight games last season and failed to win one. But in his first three starts, he threw seven touchdown passes and no interceptions in three close losses.

“He was here when we installed our system,” O’Brien said Monday. “He’s a bright guy who knows what we’re doing. We’ll watch both quarterbacks and decide pretty quick who’s going to start.”

–In Sunday’s loss at Indianapolis, the Texans played their best defensive game in the rivalry’s 13-year history. Only once — with Kerry Collins at quarterback for the Colts in 34-7 loss in Houston in 2011 — have the Texans allowed fewer than 17.

The defense was so good Sunday it limited the Colts to their fewest points this season as well as a season-worst 278 yards. The Colts entered the game second in scoring (31.3 points) and first in yards (432.4).

The defense scored the Texans’ only touchdown on free safety Kendrick Lewis’ 27-yard interception return.

“We were relentless and physical,” Lewis said. “We tried to play together, play within the scheme of the defense and make plays when they came to us.

“This hurts, but it’s like any other game in that we have to learn from our mistakes and know how to respond the next time we’re in the same situation.”

Defensive end J.J. Watt registered the Texans’ only two sacks on Luck. Watt has seven sacks in his last three games, giving him 16.5 for the season.

“There are no moral victories in the NFL,” Watt said. “It hurts regardless. We got our third-string rookie quarterback (Tom Savage) getting thrown into the fire for the first time in a tough environment. Everybody pulled together and fought, and we did a good job of that. But we needed to find a way to win, but we didn’t.”

The Texans forced two turnovers against the Colts, giving them a team-record 31. They broke the previous record of 30 set in 2004, their third season of existence.

REPORT CARD VS. COLTS

–PASSING OFFENSE: D — Ryan Fitzpatrick suffered a season-ending fractured fibula on a 2-yard run. He was 3-for-6 for 30 yards before the injury. Tom Savage, who had taken a few snaps in a kneel-down situation in his only appearance, struggled the rest of the first half but settled down and was more effective in the second. He finished with 10-for-19 for 127 yards, including perfect sideline throws of 35 and 30 yards. Then he suffered a hyperextended knee when hit on the next-to-last play. He played one more snap with the injury and threw his only interception.

–RUSHING OFFENSE: B — If Arian Foster’s 25-yard touchdown run had not been called back, he would have finished with 124 yards. As it stood, he gained 99 yards on 26 carries with long runs of 18 and 14 yards. The Colts stacked the box to contain Foster because they weren’t too worried about rookie Tom Savage beating them with his passes. The Texans ran 41 times for only 137 yards, a 3.3-yard average per carry.

–PASS DEFENSE: A-minus — T.Y. Hilton caught four passes for 50 yards in the first quarter but was shut out the rest of the way. Free safety Kendrick Lewis intercepted Andrew Luck and returned it 27 yards for the Texans’ only touchdown. Luck threw two touchdown passes but had a 76.5 rating. The defensive backs did a good job on coverage. They tried to disguise coverages to confuse Luck, and it seemed to work at times. J.J. Watt had the only two sacks.

–RUSH DEFENSE: B — The Colts ran for 110 yards, and quarterback Andrew Luck had 18. The Colts averaged 3.8 yards a carry, had no rushing touchdowns and a long run of 11 yards. The front seven did a good job of containing the run. The defensive backs played the run well, too.

–SPECIAL TEAMS: B — Randy Bullock kicked a 53-yard field goal. Shane Lechler punted them out of trouble three times and finished with a 42.7-yard gross and a 40.9 net. Punt coverage was outstanding, kickoff coverage mediocre. The Texans’ return game continues to be a liability.

–COACHING: B — Bill O’Brien and his staff, especially defensive coordinator Romeo Crennel, did a terrific job under difficult circumstances. Playing rookie quarterback Tom Savage in a hostile environment and staying within a touchdown to possibly win was impressive considering how the Colts have owned them in Indianapolis, winning for the 13th time with no losses.