Athletics lefty prospect AJ Puk will need to undergo Tommy John surgery and will miss the 2018 season, the team announced.

“Dr. James Andrews was consulted after an initial diagnosis by Dr. Doug Freeberg in Arizona, and it has been recommended that Puk undergo “Tommy John:” surgery to repair the ulnar collateral ligament in his left arm,” the Athletics said in the release.

Jon Heyman of FanRag Sports first reported that Oakland had a “concern” that Puk could possibly be dealing with a ligament issue. The “concern“ came after Puk was forced to stop throwing last week after “being diagnosed initially with biceps soreness.”

But, this injury is a huge blow to Puk and the A’s.

Drafted No. 6 overall in the 2016 MLB Draft, the 22-year-old Puk was coming off a split season with Single-A Advanced Stockton and Double-A Midland in which he went 6-10 with a 4.03 ERA and 184 strikeouts in 125 innings pitched in 27 appearances, including 24 starts. This spring, he was 1-1 with a 3.38 ERA and 1.22 WHIP while fanning seven batters in 10 2⁄3 innings pitched, but he was making a strong enough impression on the organization that he could possibly make the team this season. Puk was ranked No. 32 on MLBPipeline.com’s Top 100 prospects.

For the A’s, Puk is their second young starter to need Tommy John after 26-year-old right-hander Jharel Cotton also revealed he had to have the surgery on March 17.