LOS ANGELES — New Orleans Saints offensive lineman Andrus Peat looked mortified.

The cause of the terror: watching quarterback Drew Brees' hand slam into that of reigning NFL Defensive Player of the Year Aaron Donald.

Those fears were realized Monday. Brees tore a ligament in the thumb on his throwing hand Sunday and will need surgery to repair it, a source confirmed to The Times-Picayune | New Orleans Advocate.

Brees is expected to miss the next six weeks, the source said.

The specific ligament torn is the ulnar collateral ligament (UCL) in his thumb, according to a report from former NFL team doctor David Chao.

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Saints coach Sean Payton declined to comment Monday regarding the specificity of Brees' injury, saying doctors are "still in the midst of evaluating."

NFL.com's Ian Rapoport first reported news of the surgery.

Brees told ESPN's Mike Triplett he will meet with Tom Hunt, a Houston-based hand specialist, for a second opinion Tuesday.

"Hopefully the news is good, and the length the time — if there is any that he's out — will be shorter than longer, but again, it's a part of our sport," Payton said.

Asked how he feels about the team the Saints have built and what the other 52 players can do without Brees, Payton replied: "Well, we're getting ready to find out."

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Brees suffered his injury midway through the first quarter at the end of the Saints’ second offensive series, on a third-and-8.

Donald rushed Brees from the right side, breaking free past Peat just as Brees launched the ball, who intended to find tight end Jared Cook downfield across the middle.

TV cameras showed Brees flexing his hand walking off the field after the incomplete pass. He was unable to grip the ball on the sideline.

"Right away, when he attempted to throw, he was having pain with it," Payton said after the game.

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It's the most notable Brees injury since he missed a Week 3 game in 2015 at Carolina because of a rotator cuff injury. That was the only start he has missed to injury dating to high school.

"It's very difficult to not be playing," Brees said. "I'm not used to that. Don't want to get used to that."

Brees has been in the NFL since 2001 and has been with the Saints since 2006. In those 19 seasons, Brees has become the NFL's all-time leading passer and the league's career leader in passes completed. He has earned 12 Pro Bowl distinctions, and has been the NFL's Offensive Player of the Year twice and the Super Bowl MVP.

Rams coach Sean McVay said after the game he hoped Brees was OK, adding that sort of injury to a quarterback is "pretty debilitating."

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Rams starting quarterback Jared Goff offered his best wishes to Brees as well.

"I’m thinking about him and I told him I’m hoping for the best, and I am," Goff said. "I’m hoping that he comes back. He’s a great player for the league, the face of the league and somebody that we need in the league. I’m hoping for the best for him.”

Backup quarterback Teddy Bridgewater came in for Brees and played the rest of the game.

In Bridgewater's first significant action since before his devastating and near-career-ending knee injury in 2016, he completed 17 of 30 passes for 165 yards passing. He also rushed twice for 5 yards and was sacked twice.

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Bridgewater has been in the league since 2014. The Minnesota Vikings drafted him in the first round of the 2014 NFL Draft, and he earned Pro Bowl honors in 2015 after throwing for 3,231 yards and 14 touchdowns.

“I thought he did great in a tough circumstance," Payton said of Bridgewater after the game.

Payton declined to answer any questions regarding what the team would do without Brees, only saying: "I'm just not going to answer hypothetical questions right now. ... We'll have a plan either way."

Brees underwent X-ray imaging at the Los Angeles Memorial Coliseum. He later saw Los Angeles-based hand specialist Steve Shin after the game, according to ESPN reports.

Shin repaired the UCL in the left thumb of Los Angeles Angels outfielder Mike Trout in 2017. He also did the surgery on Los Angeles Clippers forward Chris Paul's broken finger in 2016.

Brees said after the game that he felt the injury immediately and couldn't grip the ball, adding he's never had a harder time gripping a football.

"This felt like something a bit more significant and prevented me from gripping the ball," Brees said, adding later that he was concerned. "I'm hoping it's not too significant."

He added: "I'm just hoping for the best and just preparing for whatever the next steps might be according to the evaluation."

The schedule for the Saints (1-1) over the next several weeks consists of games at Seattle (2-0), vs. Dallas (2-0), vs. Tampa (1-1), at Jacksonville (0-2), at Chicago (1-1) and vs. Arizona (0-1-1).

This story has been updated.