Let him develop, if it works out, trade him.
Subban had a lousy game in St Louis, big stinking deal.
The only thing this does is lower his value in the eyes of fans who don't watch enough hockey and can't get past superficial details.
Subban needs to play games and develop. If he does turn out to be a great goalie, there's no room for him in the Bruins organization because Rask needs a guy who's going to back him up 25 games a year. The days of Andy Moog/ Rejean Lemelin are over unless you're an unlucky team. Everyone with a good goalie rides his skill to 60+ games per season. No more platoons.
Subban, if he becomes valuable should be traded. Funny enough, he'd probably fetch a good price out of Toronto.