I'll tell you what I've told others. 75% of the bad results we had in La Liga this season were games in which Montoya was the starting RB (Osasuna 0-0, Levante 1-1, Bilbao 0-1, Granada 0-1, Sociedad 1-3 etc.). There was a serious lacking in offense when Montoya played on the right flank instead of Alves, especially in tough away games (see how few goals we managed to score in those games, under 0.5/game). So, having Montoya as the starting RB instead of Alves is a serious downgrade.
If we have difficulties against parked buses with someone like Dani Alves, wait until you see how clueless we're gonna become with Montoya/Bartra at RB. Sell Alves for someone who can step in and get the job done right away. Or else keep Alves.