We can do without a RB signing only if we drastically change our defensive strategy. With Lucho many times we lacked the so called teammate orientation, which means that when one player moves in a way his teammates move as well. This is how you obtain a fluid defense which is not based on the individual quality of each player, but on teamwork. A lot of times Roberto did not move accordingly when Pique did, or other players did not move when Roberto did. This created a lot of space for the opponent to use, simply because we did not covered the space in a synchronized way. These things are tactical. If they are sorted out nobody says we can't use Roberto at RB to great effect. We can, but we need great tactics to do it.
Let's see what Valverde has in mind. Getting the pressing back into our game is paramount.