We have relied on Fellaini in the past due to a lack of height in our team and a subsequent lack of aerial prowess. The current team has plenty of aerial ability and United are probably one of the strongest teams in the league now. Therefore, there isn't much use for Fellaini in the team any longer. I think Mourinho is coming to that conclusion as well. Blind also seems as a no contender for any of the CB spots, so that leaves us with proper CBs for those positions.
It was easy for Bailly to get a look in for a couple of reasons; one, he is bloody good, and two Jones and Rojo were injured or not trusted. Now both are playing very well. I could see the need for more centre backs, but that was before Bailly came and before our current crop started performing. It's not perfect, but as I said, I think our centre backs are the least of our worries right now. Lately, I don't see them crumbling under pressure either, and I think they've dealt with different attackers very well.