The Rockies‘ reeling rotation has taken another major hit.

Right-hander Tyler Chatwood is scheduled to undergo Tommy John elbow surgery next week and will miss the rest of the season, a major-league source said Thursday. That means Chatwood will miss most, if not all, of the 2015 season.

The Rockies already have been forced to use 13 starting pitchers this season, the most in the majors.

Chatwood, 24, and Jhoulys Chacin were counted on to be the club’s key right-handed starters this season, but the two combined for only two victories in 15 starts. The Rockies already lost Chacin because of a shoulder injury. He’s likely out for the season, but he will not require surgery.

The club had hoped to get right-hander Jordan Lyles back soon, but he was transferred to the 60-day disabled list Sunday to make room for left-handed starter Brett Anderson. Follow-up X-rays on Lyles’ broken left hand revealed it was not healing as quickly as the team had hoped. Lyles had been tentatively scheduled to begin his rehab assignment with Double-A Tulsa on Friday.

Lyles fractured the third metacarpal of his glove hand during a play at the plate June 4.

Chatwood has not pitched since April 29, when he left a game because of tightness in his elbow. The original diagnosis was a right flexor strain and inflammation in his elbow. Chatwood was making slow progress as he attempted to work his way back, but he abruptly left a live batting practice session last week, clearly upset with how his elbow responded when he attempted to throw breaking balls.

Chatwood, on the 60-day disabled list because of the elbow injury, made four starts this season, going 1-0 with a 4.50 ERA.

He also missed his first two starts to begin the season after suffering a left hamstring injury when he stretched a double into a triple during a Cactus League game near the end of spring training.

This will be the second time Chatwood will undergo Tommy John surgery. The first came when he was 16 and playing for Redlands East Valley High School in Southern California.

Patrick Saunders: psaunders@denverpost.com or twitter.com/psaundersdp