As I wrote, the first criteria isn't how he plays as CB, but it still counts to have a good DM that can cover a CB position when needed.
He isn't as good as Keita either, and as I said, I saw him play at Arsenal before, I don't really care how he did in 3-4 average games in La Liga, let's see how he does with Real Madrid or next year when we might face Bayern again or other strong teams, for now all I know is that at Arsenal he wasn't impressive, not the kind of player you want to buy if you have our clubs aspirations, and each way, he was brought as a backup for Busquets, clearly not as a replacement for that position and of course they also calculated that he will be needed as CB, that's exactly because Busquets can't, we needed a DM that CAN do a good job as CB.
So I really do hope Spurs or Arsenal bid for him and get him back to England this summer, would be perfect if done in time so the club can bring a DM that can start in many games and preferably do well as CB and be much quicker than Song, not just when Sergio is out for the day.