2 years extension year contract would make no sense from United's point of view, unless there's a clear indication (or even a clause) that it'll be extended if we improve. It basically says "I want to move to Madrid, but I expect them to have a transfer ban next summer so I need a club for another year. You're lucky enough to keep me until 2017 at which point I'll leave on a free. Adios!".
It'd be treating the player like he's bigger than the club IMO - either he signs a proper extension so we can at least get some decent money for him if he really wants to move (as Ronaldo did) or he can just leave. Trying to keep him around until the very last second possible would be a desperate move.