He's no better than any centre back currently at the club and we're not a club who can afford to pay five specialist centre backs. If we have injury problems we have options including Smith (or Ayling if he's here) subbing in, as can Howson or even Engel in dire straits plus we've retained Hannah so that looks like we've kept him for crisis situations.
If Lenihan can't recover or we lose another centre back for some reason, then we would need a 4th choice one and he would be an option but, let's face it, he'll have long since moved on to another club. 2 years ago on a free he would definitely have been a really good choice but now that time has gone.