The wide receiver tore his anterior cruciate ligament during training camp earlier this week, and is expected to miss the 2013 NFL season.
McCoy told the Chargers' website: "It's an unfortunate deal. He's worked so hard to get where he is and coming back from the other one and the season he had last year. [He was] working extremely hard in the off-season to get in great shape.
"He was in great shape coming into camp. It's the bad part of the game. I feel so bad for him because some players in this business just have bad luck. There are injuries unfortunately. It's a long season and you lose guys from time to time.
"The severity is always different but that's why we've got to look for the best 53 to be here to help us win. There are some younger players getting a great opportunity now. So it's a matter of them stepping up, and there are some big shoes to fill."
Alexander recorded 658 receiving yards and seven touchdowns for the Chargers last season.