Beckham suffered the injury in the fourth quarter after his left ankle got caught under a Chargers defender.
Beckham, who had to be carted off the field, was in serious pain and was visibly emotional as he headed to the locker room.
Following the game, the Giants weren't ready to announce whether Beckham would have season-ending surgery. That changed Monday when McAdoo made it official that Beckham wouldn't be returning this season.
The Giants coach also confirmed that Beckham will be undergoing surgery at some point this week.
"I spoke to Odell after the game. It's a sad situation," McAdoo said Monday, via the team's official website. "It's fair to say his season is over."
Beckham's ankle injury was one of several major injuries to hit the Giants' receiving corps this week. Kick returner Dwayne Harris, who also plays wide receiver, is scheduled to undergo surgery on a broken foot on Tuesday.
Beckham and Harris were both placed on injured reserve on Monday.
Two other receivers were also injured against the Chargers. Brandon Marshall and Sterling Shepard both injured their ankles against Los Angeles. The Giants announced that Shepard is now day-to-day with a sprained ankle, while Marshall will have to undergo further examination to determine the severity of his injury.
If you're wondering who Eli Manning is going to throw to this week, the Giants did add two receivers in the form of Tavarres King and promoted Travis Rudolph. King was re-signed after being released on Sept. 18 and Rudolph was promoted from the practice squad.