I believe Joe was dropped because his poor overall play and not just because of him failing to make some saves(which is a factor though). His shot-stopping ability is still one of the best in the league but more is required of a goalkeeper than that.
Goalkeepers in possesion-based teams must be good at playing the sweeper role effectively, having good distribution(something Valdes does efficiently for Barcelona) which means being able to make accurate passes to midfielders even when under pressure.
So unless Pellegrini is satisfied with Joe's play, i don't think we will be seeing him back in the team anytime soon. Although Pantilimon is not a very good replacement either.
An example: <a class="postlink" href="http://m.youtube.com/watch?v=2Kf9Wz0h7DA" onclick="window.open(this.href);return false;">http://m.youtube.com/watch?v=2Kf9Wz0h7DA</a>
Note the passes at 0:40 and 1:14