Signing a new starting caliber RB is a priority. Beyond that, I'd give Palencia a large number of minutes in preseason to at least test the waters. We need to see what his level is in comparison to Vidal overall and specifically on the defensive end (as I highly doubt he'd be better than Vidal going forward). If he proves decent enough then I'd be OK with him being 3rd backup for RB or basically share equal RB mins with Vidal. IMO Vidal's role should be a lot bigger than just backup RB, I think he would be the best backup LW/RW we have now as well and a go-to supersub given his pace and attacking abilities. If Palencia proves to be even better than hoped for, then he'd be primary backup RB and Vidal the backup LW/RW. Even in that case, Vidal should get a good amount of minutes and I genuinely feel that we could use him late in games on a fairly regular basis. If the RB is someone like Bellerin and he proves to be as bad as we fear, then the situation becomes unfortunate as we'd have 3 players capable of playing RB but none good enough to be a starter of the level we need.
I'd go for that as it seems to be playign to players' strenghts. But I'm not sure how Palencia would be able to play RB for us, if he is not very good going forward. We play in such conditions, that he would be more often in the situation to help with build-up, than in the situation to show defensive attributes.
We at Barca play an uneven game. We attack much more than we defend, considering we have the ball a large amount of time. So, imo, I expect our RB to be able to be on the ball and play it forward even in condition that would make most players play it safe. And the problem is that when you are pressured, you can only make 1 or 2 safety passes. The 3rd and 4th pass you make, if it's not advancing the play it's either a turnover or a long pass that can be picked up by the opponent's often bigger and stronger players in the air.
So, you can see that if Palencia is a standard RB, and shows no real qualities at maneuvering the ball in tighter spaces, we'll see more and more teams pressuring hard the defenders in our team who can build from the back. If Palencia is not good on the ball, going forward, then more and more teams would apply intense pressure on Pique and Busquets, as they know these are the players who can potentially take more risk in their game while also controlling the outcome of those risky situation.
So, my basic fear is that whithout a RB who demands respect on the ball, as in a player who has to be pressured close to be blocked, we would continue to struggle big time in building from the back, no matter which midfielder we sign.
I have noticed this season, that, after losing Alves, we basically have more problems building from the back because teams know full well that Roberto will not pass the ball forward except in condition of almost zero risk. He tried to build from the back while under pressure several games in a row last season (City away, Sevilla away and others), and we could see how shaky he is when he takes that responsability on his shoulders. So, if teams know that Roberto is not confident enough to advance the play, they will keep their pressure more concetrated on Busquets and Pique, our two players who have the skill and the courage to build forward.
So, if Palencia is not very good going forward (and by this I mean that he doesn't have the courage and the skill to pass the ball forward when the margin of error is small), then how he would be able to allow us to build from the back again? I'd say not being able to build from the back is one of the main reason for this season's failures.