ANAHEIM -- Angels designated hitter Shohei Ohtani will miss the rest of the season, as he’s set to undergo surgery to repair a bipartite patella in his left knee Friday in Los Angeles. The Angels announced that he's expected to need eight to 12 weeks for a full recovery.

A bipartite patella essentially means Ohtani was born with two kneecaps instead of one. Ohtani didn’t miss any time during the season with a knee injury prior to Thursday’s announcement, but it was something that bothered him at several points during the season, especially once he started ramping up the intensity of his bullpen sessions in recent weeks. The Angels were unaware of the condition until an MRI exam in February.

“This is a congenital issue that affects a small number of the population,” Angels general manager Billy Eppler said. “It’s generally asymptomatic and generally has been asymptomatic in his career, but it’s aggravated him a number of times throughout the season. My understanding is that people who suffer from this condition have a patella that did not fuse at birth.”

The news puts an end to Ohtani’s second season in the Majors, one that saw him serve exclusively as a designated hitter after undergoing Tommy John surgery on his right elbow Oct. 1 that kept him from pitching. Othani, 25, slashed .286/.343/.505 with 18 homers, 20 doubles, five triples, 12 stolen bases and 62 RBIs in 106 games. But he had trouble with his power in the second half, hitting just four homers in 53 games. One of those home runs was the hardest-hit blast of his career by exit velocity (114.4 mph) in Wednesday's 4-3 loss to the Indians.

But the injury didn’t affect his offense, Eppler said, pointing out that Ohtani's average exit velocity was almost exactly the same as last year (92.8 mph in 2019, 92.6 mph in '18) as well as his sprint speed (28.2 feet per second in ’19, 28.4 in ’18).

“Looking at some of the information and metrics, we noticed his sprint speed was consistent from last year to this year,” Eppler said. “His exit velocity was also a mirror image to last year. The only area we had to modulate was to lessen his weight room training to lessen the load. That was the only modification we had to make in-season as far as the offensive part of his game.”

Ohtani had been throwing bullpen sessions in recent weeks and was nearing an up-down bullpen session that would’ve seen him throw all of his pitches while simulating rest between innings. But now, he’ll postpone completing his throwing program until he’s done rehabbing his knee. He’ll be able to do some arm activation exercises in roughly two to three weeks, and the hope is to get him back on a mound within 10 weeks.

“We’re going to get the surgery because it started to cause Shohei some apprehension as he was increasing the intensity of his mound progression,” Eppler said. “He’s been going through those progressions and he’s been at 83, 85, 86 mph, and as he was going up in intensity, it was aggravating him a little bit. So we made the decision to kind of play it safe and not continue the mound sessions and get this condition addressed now.”

The Angels thought about waiting until Ohtani finished his rehab from Tommy John surgery to undergo the knee operation, but with it bothering Ohtani recently, they didn’t want to take any more risks, especially with Ohtani throwing with more intensity.

“We felt it was better to get it done now,” Eppler said. “We wanted to increase the intensity, and if he did that and it compromised something in his delivery, it could lead to unforeseen issues.”

Ohtani is scheduled to finish his throwing program at some point in December and will begin hitting in January or February to give him some rest. It’s unclear at this point if Ohtani will be behind other pitchers early in Spring Training next year, as Eppler said they’ll know more once Ohtani is back on the mound.

The Angels are still tentatively planning on having Ohtani pitch once a week and serve as the designated hitter roughly four times a week next year, much like the plan in 2018 that saw the two-way star win the American League Rookie of the Year Award.

With Ohtani out, the Angels can use superstar Mike Trout at DH more down the stretch, as he’s missed the past four games with a nerve issue in his right foot. Trout said he’s hopeful to return to the lineup Friday against the Rays, and the Angels now have more flexibility to keep Trout out of the outfield if his foot is bothering him.

The Angels didn’t give an update on Trout on Thursday, or on left fielder Justin Upton, who underwent an MRI exam on his right knee Wednesday.