Honestly if we dont sign anyone I do think he would have a chance of displacing Mata on the right if he takes any opportunities he gets. Its not like we're talking about a particularly high standard here. It just wouldnt be a guarantee because Andreas wasn't that great overall on the right last season either. He is at least good enough to be considered someone to compete with Mata though especially given his age vs. Mata's.
The issue with displacing/replacing Mata is that he (Mata) is also not a right winger. The right side is a problem area for us and replacing one with the other won't be an improvement as neither is good enough to lead the right side. Also, in the friendlies he played under Mou he played mostly as a playmaker rather than winger. So I'm not even sure Mou sees him as a potential RW option.
I think he needs to go the Januzaj way if he's going to find some regular football. Pick a mid/bottom table side that really wants him and prove (with some consistency) that he deserves to be playing for someone bigger.
An example of this situation is our rumoured target, Xherdan Shaqiri. He obviously had talent, that's why he played 26 league games for Bayern at 20yo. He didn't end up making it at the time (hard with Robben and Ribery ahead of him) but at 23 he left and to be fair to him he's been showing now at 25-26 that he's a cut above his teammates at Stoke. Whether he's ready for another go at a top club is debatable, but he's proven that he can be an MVP for a mid/bottom club in the EPL and he won't be short of suitors at that level, that's for sure.
Pereira's not shined bright enough to claim a spot at United (no shame, it's happened to many players) and he's approaching 23, the same age Shaqiri left Bayern. Maybe doing what Shaqiri's done could propel his career.