The three-time Pro Bowl star was injured with about four minutes left in the fourth quarter of a 27-22 loss to the Los Angeles Chargers, dropping the Giants to 0-5. Beckham had to be carted off the field after he landed and twisted his ankle underneath a hit from Chargers cornerback Casey Hayward.
"I spoke to Odell after the game," McAdoo told reporters Monday afternoon. "It's a sad situation. It's fair to say his season is over."
Beckham was the last of four Giants receivers to go down on Sunday. He finished the game with five catches, a season-high 97 receiving yards and one touchdown against the Chargers.
The team also said Dwayne Harris (broken foot) will have surgery Tuesday, Brandon Marshall (ankle) was getting additional tests Monday and Sterling Shepard (ankle) was considered day-to-day with a sprained ankle.
To fill the spots at receiver, the Giants re-signed Tavarres King and promoted rookie Travis Rudolph from their practice squad.