That defensive record has been earned by large by Bailly and Rojo. The loss of Rojo and uncertainty about his form on return necessitate that we sign at least one new CB to fill the gap.
It would be ideal if we could bring in a world class CB, but there are very few in the market and the demand is very high, with most of the big clubs in the market for reinforcements. Any world class Cb that hits the market will likely be highly over priced. We have bigger needs in other areas, and can afford to make do with a cheap decent CB, that can add to our rotation depth and possibly become a starter. Keane fits that bill. Excluding Rojo and Bailly, Keane is better than all the other 3 CB option
Jones, due to injury, in the past 4 seasons has missed 7 months, 3 months, 5 months and 3 months. That is as good as not having the player for at least half of the season. Even if he was the best CB in the world, he is worth a dime cos he is more likely to be unavaialble when needed, like earlier this season after Rojo got injured. There is no point in debating the ability of a defender who cannot stay on the field.
Smalling has been a bit better in the injury department and before this season, in which he has been out for 3 months, is only long term absence was in 2012/13. The problem with him is his slow reaction, slow acceleration, bad on the ball and penchant for blunders. He is currently the worst ranked CB (stats wise) on our team. For comparison, Bailly is one of the best in the league while Keane has similar numbers to Rojo.
Blind, as much as he tries, is not a CB. He does not have the physique, positioning, or aerial ability needed to be the starting CB at a title contender. He is fine as a last option backup, but in no way should he be part of the core rotation.
The system can make a defender look better than he actually is. Mourinho is one of the best defensive coaches in the world, and is definitely better than Dyche. If Keane looks as good as he does under Dyche, he can only be better under Mourinho. Keane has all the physical attributes and his style of play will complement Bailly well.