The San Francisco 49ers have confirmed Brandon Aiyuk has torn the anterior cruciate and medial collateral ligaments in his right knee and will miss the rest of the season.

Star wide receiver Aiyuk was injured near the end of the second quarter as the 49ers lost a Super Bowl LVIII rematch against the Kansas City Chiefs, 28-18, on Sunday.

That result dropped them to 3-4 after an injury-plagued start to 2024, with running back Christian McAffrey another big-name absentee. 

Head coach Kyle Shanahan suggested after the game that Aiyuk's injury looked serious, and the team's worst fears were confirmed when he undertook additional tests on Monday.

"I talked with him last night, just calling him at his house," Shanahan said of Aiyuk. "He was hurting, just bummed out, exactly how you'd expect. 

"But he's a strong dude and spiritual guy and believes everything happens for a reason, so he'll be all right. 

"He was definitely a little down last night, and I was just telling him how bad I felt for him and just let him know we're all here for him through this."

Fellow wideout Deebo Samuel, meanwhile, was limited to four snaps against the Chiefs after feeling unwell and was subsequently checked into hospital with a form of pneumonia.

"I think it started acting up in the middle of the night Saturday night or early Sunday morning and then it just evolved as the day went on," Shanahan said of Samuel.

"Once we found out he had the fluid in his lungs after the game, then I believe they sent him over there."

There is no timeline for Samuel's return to action, with the Niners' next outing pitting them against their long-time rivals, the Dallas Cowboys, on Sunday.

Cleveland Browns coach Kevin Stefanski fears that quarterback Deshaun Watson suffered a season-ending Achilles injury during their 21-14 defeat to the Cincinnati Bengals.

Watson will go through tests to confirm a diagnosis for his injury, which he suffered on Sunday.

But Stefanski did not have a great outlook after the game.

"Obviously, it doesn't look good with the injury," he told ESPN.

"I'll give him a call here in a little but ... disappointed for him. Any of our guys that you lose, you're disappointed because you know much this means to them and how hard they work. It's tough."

Watson's injury was sustained during a non-contact play when his leg hyperextended and he fell to the ground.

It compounds a miserable start to the season for the Browns, who are bottom of the AFC North with a 1-6 record.

Fan frustration was noticeable, with many seeming to cheer Watson's injury after he had been booed during the pregame.

"We don't boo guys that are injured on the field, especially when the cart comes out," said defensive end Myles Garrett said after the game.

"We should be ashamed of ourselves as Browns and as fans to boo anyone and their downfall."

Stefanski was in agreement with Garrett.

"I don't think it's ever OK to cheer when someone's injured. I'm sure it's not every person in the building doing that, but that's disappointing," he said.

Meanwhile, it is feared San Francisco 49ers receiver Brandon Aiyuk suffered a torn ACL during their 28-18 defeat to the Kansas City Chiefs on Sunday.

"That's what it looks like, but we don't know for sure," coach Kyle Shanahan told ESPN. "Been wrong before. So, praying that we are [again]."

Further tests will confirm the exact issue and determine whether Aiyuk will miss the rest of the season.

With fitness problems mounting for the 49ers, Deebo Samuel was also announced to be suffering from illness before kick-off but was still expected to play.

He was withdrawn after four snaps with no catches or carries.

"Throat, stomach things, just real fatigued," Shanahan said, on Samuel. "Struggled to breathe, couldn't catch his breath ... he kept trying to fight through it, but once he was struggling with the breathing and everything, we had to shut him down."

Aiyuk's teammates, meanwhile, were vocal in their disappointment at potentially missing the wide receiver for the remainder of the season.

"It sucks," said tight end George Kittle. "I love Brandon Aiyuk, the energy that he brings, what he can do on the football field, routes, catching the ball. He's an incredible football player. You can't really replace, in my opinion, one of the best receivers in the NFL."

Quarterback Brock Purdy echoed Kittle's sentiments.

"What BA does for our offense and who he is and as a teammate and the brother of ours, man, it's sad," he said.

"Wish him nothing but the best and praying for him. So that's my guy. And when he goes down like that, obviously we have other guys that can step up and do their job really well ... we all just got to come together as a group collectively, the receiver unit and all of us."

San Francisco 49ers receiver Brandon Aiyuk is currently in a contract battle with the team, and Fred Warner believes it is a "good issue to have".

Aiyuk was in contract negotiations with the 49ers to extend his current deal but reportedly has not held talks with them since May.

Unable to reach a deal, the 26-year-old instead asked for a trade.

It is not the first time the 49ers have found themselves in lengthy contract disputes, going through the same process to resign Nick Bosa, Deebo Samuels and Warner himself.

The linebacker was unfazed by the controversies though, instead choosing to focus on the positives.

"When you're part of such a talented roster, I feel like this is something that goes on every season," Warner said via NBC Sports Bay Area.

"It's a good issue to have because, obviously, that means that your players are performing at such a high level that you have to have these kinds of conversations."

Aiyuk is under contract with San Francisco for 2024 in the option year of his rookie deal.

He impressed last season on their run to Super Bowl LVIII, earning second-team All-Pro honours and ranking seventh in the NFL with a career-high 1,342 receiving yards.

His average of 17.9 yards per catch ranked second in the league, while only the Miami Dolphins' Tyreek Hill bettered his 16 receptions of 25 or more yards.

Overall, since being picked in the 2020 draft, Aiyuk has 269 receptions for 3,931 yards, with 25 touchdown catches in 62 games.

As a key figure in the team, Aiyuk has been supported by his team-mates throughout the dispute, and Warner said he is ready to welcome him back when the saga is sorted.

"It's a part of the business, and so we all just let them handle that. And when he's back, he's back. We'll welcome him with open arms."