ANAHEIM -- Shohei Ohtani took another step forward in his recovery from Tommy John surgery on Saturday, as he threw curveballs in a bullpen session for the first time since undergoing the operation on Oct. 1.

Ohtani, who is aiming toward pitching again in 2020, threw a 40-pitch bullpen session before Saturday’s game against the White Sox, and estimated that he threw his curveball 10 to 12 times. He’s still not quite ready to throw his splitter or slider off a mound, but he has been throwing them on flat ground.

"It went really well overall," Ohtani said through interpreter Ippei Mizuhara. "Pre-surgery, the curveball hurt me the most of all my pitches. But after the surgery, after throwing it today, it actually felt more comfortable than the fastball. So that’s a really good sign."

Ohtani, though, wasn’t throwing his curveball with full vigor. But he’s been increasing his intensity with his fastball, as he reached 85 mph with it on Saturday.

"He was throwing breaking balls extremely cautiously," Angels manager Brad Ausmus said. "It’s not like he was snapping off sliders. He was throwing nice and easy breaking balls. He did put a little effort into the fastball, which was good to see, but we still have a long way to go.”

Ohtani will continue his throwing program until he’s ready to face hitters in a simulated game, which should come near the end of the regular season. The plan is to get Ohtani fully through his rehab process this year, so that he can have a normal offseason and come into Spring Training ready to be the two-way player he was in 2018, when he won the American League Rookie of the Year Award.

“The more I throw, the better I feel with my mechanics, and the ball coming off my fingers," Ohtani said. "So I feel a lot better than the beginning of the season. So I feel like I’m making a lot of progress.”

Ohtani, 25, wasn’t in the lineup against the White Sox on Saturday, and has struggled with his power in the second half. He's hitting .282/.360/.400 with one homer and 12 RBIs in 32 games since the All-Star break.

He's struggled to lift the ball in the air this year, as he's seen his average launch angle decrease from 12.3 degrees in 2018 to 6.1, per Statcast. He's had the same average exit velocity of 92.6 mph in both seasons, but he posted a .564 slugging percentage in 104 games as a rookie compared to .510 in 85 games this year. He's also seen his isolated power (slugging percentage minus batting average) drop from .279 in 2018 to .214 as a result of not getting the ball in the air enough. The league average ISO is .183, so Ohtani still has above-average power.

“I’ve been making contact," Ohtani said. "Just missing it by a little bit a few times. So I’m getting closer. Popping up some I shouldn’t be, but I should be coming together soon. I feel good plate at the plate. I’m just not hitting it at the right angle. That’s the biggest thing.”

Pena undergoes successful surgery

Right-hander Felix Peña underwent season-ending knee surgery to repair a torn ACL in his right knee on Friday. Pena, who suffered the injury while trying to cover first base on Aug. 3, finished the year going 8-3 with a 4.58 ERA and 101 strikeouts in 96 1/3 innings.

“The report is it went well, but I’ve never had a report tell me it didn’t go well, so take it with a grain of salt,” Ausmus said. “I’ve never had someone come out of surgery and say, ‘That was awful. Guy botched that one.’”

Middleton nearing return

Reliever Keynan Middleton (Tommy John surgery) started his rehab assignment with Triple-A Salt Lake on Friday, throwing a scoreless inning with one strikeout. Middleton is expected to need one or two more outings before he’s reinstated from the 60-day injured list.

“He said he felt, quote, 'Amazing,' after his outing,” Ausmus said. "Assuming everything goes smoothly, [he needs a few more appearances]."