Sounds like trouble. We have players like Mascherano and Cesc who can tackle in midfield and intercept if need be and even Song who can perhaps add to physicality and muscle in the middle of the park. If Tata needs to change up some tactics, I'm of the opinion that he can do it with his current roster.
Also, with Alexis, there is no need to disturb something that has taken 2+ seasons to finally get functioning. I can see the need for a CB, GK, CF, maybe recruiting one or two B players, and to offload a few players, but I don't see the need for Vidal, as talented as he is. Plus, you can't assume Vidal will come here and just slot right in with the Catalan group and integrate without a transition phase. He appears to be a player that would have a hard time getting on with some of our players, who are the 'nice' type. He doesn't seem like a nice guy (which is not a slight to him by the way.)