An already short-handed Bears defense — playing without starters Akiem Hicks, Danny Trevathan and Prince Amukamara — was dealt another major blow when linebacker Roquan Smith suffered a pectoral injury in the first quarter against the Cowboys on Thursday night.

Veteran Kevin Pierre-Louis replaced Smith and played a key role in a stellar defensive performance against the NFL’s

No. 1-ranked offense in a 31-24 victory at Soldier Field. But Smith’s injury is likely to end his season.

“It doesn’t look real good for him,” Bears coach Matt Nagy said.

Smith’s loss is a big one. The second-year linebacker from Georgia was returning to form after a midseason lull that coincided with his absence against the Vikings in Week 4 for a personal issue. Smith had 15 tackles and two sacks against the Lions on Thanksgiving.

“Man, it sucks. It really sucks,” safety Eddie Jackson said. “You hate to see one of your brothers go down, but especially a guy like Ro. It was just kind of like a shock to the stomach.”

Smith suffered the injury on the Cowboys’ 17-play, 75-yard touchdown drive to open the game.

“He told us in the game — he thought he hurt his pec. And I was just like, ‘Dang,’ ” Jackson said. “He said he’s going to finish the drive. You could see his face. I’m like, ‘Ro is still going to fight through it.’ And he fought through it. It just sucks. Ro’s having a good season. Unfortunately, it gets cut short with the injury.”

As well as Pierre-Louis played in his stead, there’s only one Roquan Smith. And with Hicks expected back against the Packers on Dec. 15, the Bears were hoping to get a step closer to full strength.

“It could be a big loss,” Nagy said. “The way that Roquan has been playing the last couple weeks — flying around, making plays — you just love that confidence that he’s bringing to the defense.

“[But] the depth that Ryan [Pace] and his guys have created on this roster allows somebody like Kevin Pierre-Louis to step up and make plays. You feel bad for Roquan, but we have guys that can fill in.”

As it did against the Vikings without Hicks and Smith in Week 4, the Bears’ defense stepped up with key players missing.

Kevin Toliver played for Amukamara and Pierre-Louis replaced Roquan Smith and the Bears didn’t miss a beat. They actually were statistically more effective without Smith than they were with him as they built a 31-14 lead in the fourth quarter.

After the long opening possession, the Bears held the Cowboys to 58 yards on 19 plays on their next five possessions in building a 24-7 lead after three quarters.

“We’re just a resilient team,” linebacker Leonard Floyd said. “A team full of fighters, and we’re going to go out and fight every game to win. It’s the group of guys we’ve got in the building. Everybody has a next-man-up mentality. And guys are doing a good job of coming in and playing hard.”

It was Pierre-Louis’ turn to step up Thursday night. The six-year veteran had five tackles, one tackle-for-loss, one quarterback hit and two pass break-ups against the Cowboys. When Smith went down, he had no hesitation.

“In those moments, there’s no time to think like that,” Pierre-Louis said.

“You’ve just got to be there for your brothers and then just go with what you’ve been practicing each week. I’m blessed to have the guys round me to help me out if I need help — guys rushing; guys covering. I just need to do my job.”