Surprised that no one mentioned a rumor that I've heard in several french and brazilian medias (but maybe spanish websites didn't talk about it at all): Pablo, from Bordeaux.
CB, 27, Brazil NT player, best defender in the brazilian league in 2017, and probably one of the best CB in Ligue 1 this season. Rumor has it that United and Barca want him.
He was bought by Bordeaux in 2015 after a very good season in Brazil (apparently) but he got injured quickly, and failed to adapt to Ligue 1. Was then loaned to Corinthians for a full year, where he won the league and was, as mentioned, voted best defender. Came back to Bordeaux last January, and he was completely changed (he said that his coach with corinthians helped him improve drastically). He's one of the main reasons (with our other CB, Jules Kounde, very promising 21yo) why Bordeaux went from the 15th place in december to the 6th in june. Krasnodar apparently offered 12 millions for him in august, but the player refused to go there. He's been very good since the beginning of this season, which allowed him to play twice for the brazilian NT.
He's a big guy, but not too slow for his size and weight, and even though he's no Beckenbauer, he's actually rather good with the ball.
Not saying he would be a perfect fit, just thought it was curious that nobody mentioned him here. I think he's good enough to be a backup CB in a top team, but I'd be surprised to see Barca pay 15 millions (his alleged prize right now, which doesn't shock me considering the current market) for, well, a backup CB who's already 27.