The San Francisco 49ers improved to 3-0 on the young season, defeating the division rival Arizona Cardinals, 16-15. However, it might prove to be a pyrrhic victory as All-Pro edge rusher Nick Bosa went down with an injury.

Following some tests done on Monday, it has been determined that Bosa suffered a torn ACL, per ESPN's Adam Schefter. His season is officially over. This is the second time that the future Hall of Famer tore his ACL. He previously suffered the same fate in 202 against the New York Jets.

It's just another blow to a team that has been dealing with injuries all over the field, going back to training camp.

In Week 1, the 49ers lost their starting quarterback. Brock Purdy left their season-opening win with turf toe, but is reportedly nearing a return. In his absence, Mac Jones has played exceptionally well. That likely has much to do with head coach Kyle Shanahan.

Wide receiver Brandon Aiyuk won his contract battle with San Francisco, and was given a four-year, $120 million contract. But last year, he got banged up and was placed on the PUP (physically unable to perform) list to start this season. He will not return until Week 7 at the earliest.

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The wide receiver room took another hit when Jauan Jennings suffered an ankle injury. He missed Sunday's game and is questionable for Week 4. Thankfully for 49ers fans, second-year pro Ricky Pearsall has stepped up big in his absence.

But now even the defense will be without its best player.

Before going down, Bosa had two sacks and nine tackles this season. For his seven-year career, he has 64.5 sacks, earning Pro Bowl honors five times and was named Defensive Player of the Year twice.

Considering the 49ers are playing in what, at this point, is the best division in football, this loss might be very costly.