Rafael is comfortably better than him. Clyne can stay fit, plays in a team with good defensive tactics with player like Bertrand picking up clean sheets for fun. Even Yoshida looks quality there. Another Clyne's advantage is that he can play on the left as well but Rafael doesn't look out of pace there either.
I guess we are well sorted with Valencia making solid impact and with almost no injuries or dips of form is worth to keep, Rafael still 24! as best RB when fit needs to be given chance like Herrera who proved van Gaal very wrong. They are actually having similar season but Rafael didn't get a chance yet.. Also the best RB in next ten years can be the third option here - McNair and Young is also right footed to be a solid backup, we absolutely don't need to splash money for this surely overpriced player in the summer, he's not even better than Valencia imo.
On the left we have Shaw, Young, Rojo and James as backup backup, so we don't need fullback at all
Also I forgot to mention versatile Lingard and Varela who has big potential however he will end up in Madrid unfortunately.
SO again no need for FB, spend elsewhere! fecking DM and wide attacker