That seems strange. Perhaps it is a case a mutual agreement to terminate his contract, rather than retiring (unless he has picked up an injury). Why would he restrict his access to his livelihood? There must be some clubs who desperately need back up in central defence. Back with us in a couple of weeks or Man Utd!
He's 34 (35 in Feb). I think it's age and injury that's restricted his access to his livelihood. Not uncommon for some players to jack it in if they can't keep to the standards they set themselves, both personally and club level-wise. Probably thought a nostalgic stint back in Belgium would be fun but found it anything but. Plus Northern Ireland aren't qualifying for the Euros either and there's no way he'd be in the running for their WC quali squad. All in all it's probably time to quit.
Apparently he's found it hard to motivate himself to continue being a professional...maybe a good thing we didn't retain him after all!