Shohei Ohtani will miss the final 15 games of the season after the Angels announced that he will undergo left knee surgery on Friday to correct a congenital condition that has been bothering him occasionally since February.

The timetable for recovery from this surgery is eight to 12 weeks. The Angels expect Ohtani to continue throwing a few weeks into that, and still be able to conclude his pitching rehab from Tommy John surgery in December, General Manager Billy Eppler said.

Ohtani has a bipartite patella, the Angels said. That means that his knee cap never developed normally as one bone, instead remaining split into two. It’s a rare condition that affects less than 3 percent of the population. Eppler said it is often without symptoms, and Ohtani had not mentioned it bothering him in his career prior to this year.