We've already got 5 CBs in Jones, Smalling, Rojo, Bailly and Lindelof. There are also TFM and Tuanzebe that can play as CB, if we won't start playing them, they won't develop. Bailly - Rojo partnership sounds the best possible, Jones is also in top form this season. The truth is with left footed CB we probbably wouldn't concede an own goal (look the way Phil clears it). Mou is getting a full back in the summer and I'm 100 % sure about it.
Problem behind some goals might still be in our midfield. I just have to write down Pogba. The situation is that he doesn't make as much recovery runs as CDM should when he's playing at that position. Matic isn't that fast so he can't cover all the space in front of defensive line. I'd love to see someone who can play with Matic and is faster, maybe smaller then him, but phisically strong - I can't see Herrera beeing the one. Pogba has to play in 433 midfield or as number 10 in 4231 - no need for aditional CB, we are good right there.
EDIT: It looks like Chiellini will be available in the summer (according to their president), since Milan doesn't stand a chance to get in CL. I'd take him if there's any chance for us.