I looked at him as a sleeper coming into this season. I don't know how great his numbers will be but from what I have read he does have a little potential for power, as you said. Basically, I think you answered your own question. He is doing well, as I expected, in 3 categories, and power could show up later. I did expect a little more speed from him but you can never be 100% sure with players who are just breaking into the bigs.
At this point he is doing 3 cats well so ... If it is a keeper league, I would try to stash him in your lineup. In non-keepers, I think he warrants playing time if you are in a deep league or really need outfielders.
Leyland said, "We thought we were getting a hell of a player, but Neifi simply did not perform well."