OAKLAND -- Sean Manaea 's second big league season didn't go according to plan, but the A's still view the big lefty as a frontline man in the making.

Already, Manaea figures to slot into the No. 2 spot in the rotation behind right-hander Kendall Graveman , and there's a thought that his potential could soon translate to ace status.

"I think a healthy Sean Manaea is a guy who is going to pitch at the top of anybody's rotation," A's manager Bob Melvin said.

Manaea recorded a 3.76 ERA in 16 starts leading up to the All-Star break last season, an encouraging stretch that was briefly interrupted by a two-week stint on the disabled list because of shoulder issues. But the beginning of August brought forth struggles, and Manaea finished the month with a 9.17 ERA.

Diminished velocity was at the root of the issue, which could be traced to troubling weight loss. In September, Manaea acknowledged that the problem stemmed from ADHD medication he was prescribed in Spring Training after being diagnosed in the offseason. While trying to pinpoint the right dosage, Manaea's appetite deserted him.

Food became less appealing, and the 6-foot-5 pitcher dropped 25 pounds.

"That was a big factor," Manaea said at the time. "I know that's not healthy at all, especially with it only being a couple months. I definitely noticed it halfway through the season when I was having a hard time recovering."

Manaea said he weighed 230 pounds in late September and noted his desire to add more muscle and reach the 240-245 range in the offseason.

"255 is a little too heavy and 230 is not enough," he said. "I'm trying to find that balance, and being able to get through an entire season is going to be huge."

"I think he was a little bit worn out at times last year," A's manager Bob Melvin said. "It was a long season for him, and you could see the velocity was going down at times. And he had to deal with nagging injuries for the first time in his career, too."

There's a good chance Manaea emerges better for it. He showed signs of this in September, posting a 3.54 ERA in his final five starts while learning to pitch effectively without his normal velocity -- an attribute that typically accompanies the best of pitchers.

The strikeouts returned, with Manaea tallying 23 in his final 28 innings to bring his career total to 264 -- third most in Oakland history by a pitcher in his first two seasons in the Majors.

Manaea, who will turn 26 in February, was a first-round pick by the Royals (34th overall) in 2013 and came to the A's in the 2015 Benjamin Zobrist trade. His continued development and budding dominance will be integral to Oakland's 2018 success. With a potent lineup intact, the A's need their pitching staff -- a weak link for the 2017 group -- to keep up.

"I think the upside is still there," Melvin said. "For the first time in his career, he went through a tough period. I think he's going to be better for it ... to look through some struggles, look at things differently, maybe video, maybe pitch selection, stuff that came pretty easy to him for a while."