Blind played at CB when he had Fellaini there to protect him. Since Fellaini's been dropped, so has Blind from centreback.
Meanwhile, Blind has played at left back 6 times. On only two of those occasions have either Martial/Rashford played in front of him, despite them being the most frequent starters at leftwing. In fact, Blind has played leftback EVERY SINGLE TIME someone other than Rashford/Martial has been on the leftwing (when Lingard, Rooney, Young and Mkhitaryan started there).
It's pretty clear that Mourinho likes Blind at leftback. Indeed, he's first choice depending on who plays at leftwing. But the manager doesn't trust either Martial or Rashford to give him the protection he needs.
That's always the problem with Blind. He needs someone in front of him whether it's in the middle or on the flank. He simply lacks the positioning and pace to go with his great passing and crossing ability.