Moss has been dealing with the issue for much of the season and was unavailable Wednesday in the A's 5-4 loss to the Los Angeles Angels.
An MRI showed a significant amount of torn cartilage, to the point of bone-on-bone problems.
“It was worse that we thought it would be,” said Moss, who had an MRI and a cortisone shot before the game. “The shot should help me get through the rest of the season, but it won’t fix it.”
Moss could be available to play as soon as Thursday or Friday.
The All-Star first baseman is hitting .237 with an OPS of .779 and 25 home runs this season but has hit just .179 with a .592 OPS and four home runs in the second half.
- Ben Estes