Joey Gallo, 1B/OF
Unfortunately, this one feels rather obvious. Gallo had a much better year than anyone thought he might and it was definitely a bounce back from his dog days in the Bronx. It still wasn't great, though, and Gallo fizzled out as the season went on to the point where he wasn't even considered for the postseason roster.
That's not exactly what you're looking for out of a guy who is being paid $11 million.
Gallo started the year pretty strong, but he might have peaked during his revenge series against the Yankees at Target Field. During that series, Gallo hit .375 with 3 RBI, a 1.694 OPS, and was heckled by Yankees fans for playing too well. His whole first month was pretty solid, as he slugged 1.066 with seven home runs and 14 RBI.
Things went downhill from there, with Gallo's numbers dipping each successive month ending with him only playing five games in September. Injuries didn't help, but it was a steady downward trajectory after his hot start to the point where fans were calling for him to be DFA'd. That didn't happen but Gallo did himself no favors in auditioning for another contract with the Twins.
It was a good gamble by the front office, but it didn't work out the way anyone wanted it to.