Keane is good and he's going to be very good. I've been singing his praises ever since he partnered Wes Morgan at Leicester while out on loan. Leicester didn't go up that season but they did have the best defensive record in the division. I said not long after that, that he'd one day play for England, and I was astounded when we let him go for such a pathetic fee.
With all that said, he isn't what we need right now. We've already got four good centre halves that defend well. We already have one of the best defensive records in Europe, which will remain even when we inevitably improve the attacking side of our game.
If we're going to buy a centre half this summer - which is certainly practical given that Rojo is out long term and Jones just can't stay fit - then this player needs to be better than what we have. Keane isn't that player. I don't think he'll ever be better than Smalling and Jones (when fit) either.
I know that pretending as if both Smalling and Jones are morons is in, but they're a hell of a lot better than the credit they get. They aren't protected by performing relatively well at a Burnley, or a Southampton. They're held to a much higher standard. Honestly, plonk the pair of them down elsewhere in the league and many of the same people that criticise them (and some of the very same people that would likely want Keane to return) would be full of praise for the dominating performances they would put in at a West Brom, or a Stoke.
I've got lots of time for Keane, like I said. And if he replaced Jones on the basis of fitness then that's kind of understandable. But he isn't better defensively, not a chance. If we're buying defenders then they've got to improve the first team. If not, then what's the point?