San Francisco 49ers Pro Bowl edge rusher Nick Bosa left Sunday’s game against Arizona with a right knee injury, from which he did not return.

Initial tests done by the team’s training staff came back negative, but after an MRI on Monday, it was reportedly determined Bosa tore his ACL and will miss the rest of the 2025 NFL season, according to ESPN’s Adam Schefter.

Bosa was off to a hot start this season, already having two sacks in San Francisco’s first three games. He left Sunday’s win over Arizona late in the first quarter with two tackles and a forced fumble. The All-Pro defensive end left the field and was seen motioning to the stands with a thumbs-down signal as he was led to the locker room.

Serious knee injuries are nothing new for Bosa as he missed the majority of his second year in the NFL after suffering a torn ACL during Week 2 of the 2020 season. Sunday’s injury occurred in his same knee, marking the second time Bosa has torn his ACL in his right knee.

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The 49ers finished that season 6-10, missing the postseason after making it to the Super Bowl a year earlier. Bosa returned from his ACL injury in 2021, to help the Niners make it back to the NFC championship game where they lost to the Rams.

In 2021, Bosa started all 17 games for San Francisco, made his second Pro Bowl in three years and recorded 15.5 sacks. Since entering the league in 2019, Bosa has the fourth most sacks in the NFL. He led the league in sacks during the 2022 campaign while being named first team All-Pro and Defensive Player of the Year.

The 49ers are not the deepest team in terms of edge rushers backing up Bosa. Right now, Bryce Huff and Sam Okuayinonu are in line to step in for Bosa. As one of the leaders on the defense, this is a huge blow to the 3-0 Niners and will have a major effect on the team’s pass rush.

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San Francisco is already missing stars on offense like quarterback Brock Purdy (toe) and wide receiver Brandon Aiyuk who hasn’t played since last October when suffered ACL and MCL injuries ending his 2024 season prematurely. Add Bosa to that list of star players sidelined with significant injuries for the 49ers.