Sean Manaea's big-league debut for the Oakland Athletics didn't go smoothly at first. Through his first 67 innings pitched in 2016, the southpaw struggled to a 5.24 ERA to go along with a batted-ball profile that included a 22.9% line-drive rate, 37.1% fly-ball rate, and 34.6% hard-hit rate. But still, a 4.43 SIERA told us things were bound to get better, and they did.

He finished his rookie season with a flourish in the second half, posting a 2.67 ERA (3.67 SIERA) over his final 77.2 frames. Opposing hitters managed just a .276 wOBA after having that number up at .338 prior to the All-Star break.

These good vibes mostly continued for Manaea prior to this year's midsummer classic, too -- he boasted just a .290 wOBA allowed and 4.09 SIERA through 93.1 innings. Unfortunately the last month and a half has totally put a crimp in his style.

The left-hander has posted a second-half ERA approaching 7.00 in his most recent 37.1 innings, which has lead to an overall ERA of 4.55 and SIERA of 4.43 so far this season. One noticeable change from Manaea's rookie and sophomore campaigns is his pitch usage -- he's throwing his fastball just as often as last year (58.1% in '16, 58.3% so far in '17), but the balance between his slider and changeup has shifted.

Below is a table that shows how his usage between these two offerings has changed, along with the wRC+ allowed in each season.

Clearly, the more drastic change going on here is with Manaea's changeup. After limiting opposing hitters to an .088 ISO on that pitch last year, they currently own a .230 mark. After building a lot of positive momentum, he now has some work to do before winter hits and 2018 is officially on the horizon.