I don't see why signing him would mean that Mickey doesn't get a shot.
Injuries create minutes over the course of the regular season, but in the playoffs a veteran replacement-level 10th man is a useful insurance policy. More so than a rookie.
I'm not saying I think Thompson is our best option, but we're the #3 seed right now and with Danny standing pat at the deadline, kicking the tires on guys like Thompson seems reasonable.
And if we can get someone who's a better player than Mickey (or anyone else on the roster) right now, I think you make the move.
As an aside, I think we are shaping up to be that team no one wants to meet in the playoffs. Tough, defensive-minded, capable of offensive bursts, well-coached...it's pretty cool.