I don't think anyone would pick Sali over Candreva because he's only on loan. I can understand you would pick a similar, young player who's a Juventus player over someone on loan, but not in this case.
Sali is not getting any younger and that wouldn't be a problem if it weren't for him getting worse and worse. He keeps disappearing in games and adds nothing to our midfield. Candreva, who's definitely not a world beater, could be better than Sali and there's plenty of room to improve.
Ok, he's only on loan, but we just need results now. Zac probably won't even be here next year and Sali definitely isn't in future plans, so I don't think that's the problem.
The only reason I can think of would be that Sali is probably better defensively.