It truly amazes me when I hear or read someone saying that we need another right back. I agree with Mourinho, Antonio Valencia is a world class Right Back and what he provides up the flanks, makes us a different team and allows us to play a far more attacking style which in turn, brings out the best in our other skill players.
I'll take this one step further and say that it's far far easier for a team to reach the highest levels of performance if they have properly functioning attacking full backs. This is just the style of football that I believe in and I think coaches that play flat back 4 and encourage their backs to be just defenders are wasting massive possession and attacking opportunities.
My son plays RB at a pretty high level here in the states and we are always discussing the position as we watch games together and more times than not, I use AV as the example of how to play the position properly. As a matter of a fact, he plays even better than those Brazillian full backs that fancy themselves as wingers. AV knows that his job is to also and foremost, get back and help play defense.
IMO:
First a right back must be a good defender. And at times, Valencia is a great defender. He's not afraid to win a ball in the air and he is extremely strong, not to be pushed off the ball and he takes it personally to not allow a cross from his corner. He is also very fast which is an extremely helpful trait. In short, he's a physical specimen.
Second, I believe a world class right back should have a massive work rate. Check!
Third, He should be willing, able and skillful at helping out on the attack. Check!
Additionally, AV is a leader. Wearing the Captain's armband last game!
So, at least for a couple more years, I don't think we need to consider spending a dime on a RB of the future. We have him in the lineup. And we have Fousa Mensa waiting in the wings as a potential player of the future. Darmian is a reasonable backup IMO.