Well his OPS and batting average have dropped every single year, there is no discounting that. His slugging has remained the same, but his OBP has dropped every year.
A couple of things. Batting average will always fluctuate due to BABIP. It's not something you can really control. You can generally control how much power you have and how much you walk. Raw stats like OBP and OPS. Those will drop if your batting average drops. It's all connected that way. But Mac's isolated slugging and isolated OBP (which is taking batting average out of it) has remained consistent. That's really what you are looking for.
Not to mention that raw numbers aren't the greatest thing to look at as the league shifts to a more pitcher dominant league. That's what league/park adjusted stats like wrc+ and ops+ are much better to look at. And that has been extremely consistent. If Mac goes to Texas next year I would expect his raw numbers to take a jump due to the park. But I would bet his overall numbers would leave him around a 120 wrc+. He is going to be a great fit for an AL team.