He hit .261 with a .343 OBP, .871 OPS, and 11 HRs in 157 ABs as a 22 year old. He also showed a lot of patience at the plate and looked mature beyond his years. Factor in that his team plays in a great hitter's park, with a stacked lineup, and those are the reasons that many are so optimistic as to his outlook.
Unfortunately, he really hasn't been able to stay healthy since he put up the above referenced numbers (back in 2015). He missed all of 2016 with a shoulder issue, was arguably the best hitter in the league during Spring Training in 2017, then started having heel and ankle problems as the regular season began, I believe resulting in two separate surgeries last year. He had more issues with the heel and ankle this Spring, neccessitating yet another surgery.
The injuries are obviously a problem, and he'll need to show he can stay healthy or he could go the way of Nick Johnson. But the talent is real, and the opportunity for him to seize the 1B job and put up big numbers is there. He may not ever win a batting title, but he's not a sub-.200 hitter either.