Westbrook was placed on the disabled list Tuesday, retroactive to May 29, and Indians trainer Lonnie Soloff said Saturday he will have surgery to repair a torn ulnar collateral ligament in the next week or two.
"If you pitch long enough at a high level, you are going to experience some kind of an arm breakdown, whether it is your shoulder or your elbow," Soloff said.
Westbrook was examined Friday in California by Dr. Lewis Yocum, who will perform the surgery.
"We all had a pretty good idea that something serious was going on, but you always hope for the best," manager Eric Wedge said.
Westbrook went 1-2 with a 3.12 ERA in five starts this season. He was sidelined for about a month with a strained oblique muscle, then felt pain in his elbow after his first start back.
The Indians entered the year with one of the deepest rotations in baseball but have been hampered by injuries and were 6½ games back of the AL Central-leading White Sox entering play Saturday. If they fall further off the pace, they could look to trade left-hander C.C. Sabathia, the reigning Cy Young Award winner who is eligible for free agency at the end of the season.
Westbrook, who will be out for about a year, will be replaced in the rotation by Jeremy Sowers. Another Cleveland starter, Fausto Carmona, is expected to remain sidelined for at least two more weeks with a hip injury.
"It goes back to starting depth," Wedge said. "We have it and we've had to use it."