Obviously, how things go over the next six weeks will play a role in Tabata's placement. However, so, too, will Andrew McCutchen's fate. Tabata and McCutchen are both natural center fielders, and Pirates management does not want to move either from that position.
That means that if McCutchen starts the year in Triple-A, which is where he is projected to begin, Tabata will start the season back at Double-A Altoona and remain there until McCutchen gets the call to the Majors later in the season.
However, if McCutchen surprises and makes the team out of Spring Training, then I'd expect Tabata to begin at Triple-A.
That clears things up somewhat. Based on what I've read about Tabata's defense, though, he's not going to be a centerfielder in the bigs, so I wouldn't worry much about putting him and McCutchen on the same team, if that is otherwise the best thing for both of them. Of course, if they aren't at the same level, I'd play them both in center, but Tabata is becoming a big man, and he probably will have to move to a corner soon.