Valid point.
It's just at UTD in particular, unless Moyes moves away from the traditional 4-4-2 which he could, I think he will struggle more than most. All of those players you listed are either strikers or in Mata and Martinez's case, had very little competition in the squads for how they played. Strikers can have it easier as all they have to do is score, as long as they do that, then they are fine.
At UTD, Thiago will have to compete with Carrick, Cleverley, Kagawa and Giggs with Kagawa in particular giving him some stiff competition. I wish Moyes would realize that he already has a world class CAM in the form of Kagawa and if they allowed him to play in his natural role, they wouldn't need Thiago or Isco. UTD need a DM more than anything, a hard-nose ball winner who embodies the spirit of ole captain himself Roy Keane (always loved him as player).
Point being, its bc of UTD's style of play and the squad they have already that makes me skeptical of how well Thiago would do there. Arsenal on the other hand would be a completely different story, if he went to Arsenal he would be better than Fab was there, guarantee it.