Cotton was diagnosed with a sprain and had a second scan taken on his elbow on Friday. Then he was told he would need Tommy John surgery. The procedure is expected to take place next week.
"It's tough. I'm missing the 2018 season with my boys, so it's kind of hard to take. I'm trying to take it as best as I can and just get ready for the long process, the long road that's ahead," Cotton said Saturday morning at A's spring training.
The 26-year-old Cotton was expected to be in Oakland's starting rotation this season after going 9-10 with a 5.58 ERA in 24 starts as a rookie in 2017.
He pitched four times this spring, with a 3.75 ERA and 16 strikeouts in 12 innings.
"Unfortunate. Just of him having to go through what a lot of guys have to go through,'' A's manager Bob Melvin said. "But we felt like he was on the cusp of really coming into his own, and it's going to be delayed a year, unfortunately."
The A's figure to go into the 2018 season with Kendall Graveman, Sean Manaea and Daniel Mengden in the rotation. They are still evaluating other starters, but Cotton's loss means more decisions for the rotation and for a possible long reliever.
"There's a trickle-down to the Cotton effect,'' Melvin said. "It was looking like he would be in our rotation, for sure. Someone else is going to have to step up as far as that goes. Once we decide the five, then we'll take a look at who our length would be."
Manaea said losing Cotton hurts.
"He's going to come back, and hopefully things will work out and not have too many setbacks," Manaea said. "Losing him is a pretty tough blow, but I know we've got guys that are going to step up and perform.''