Crikey, that's a surprise!
I'd guess that as they're registered as our players, signing before the deadline, there are no requirements to have played a set number of games to be able to play in the play-offs.
Ideally, we need them playing as soon as possible, not just turning up for the first play-off game.
I suspect that it will have cost a few quid for the Dons to have done this as they won't be coming here for peanuts.