Usually in the NHL, there is a number 1 and a number 2 between the pipes. When the primary goaltender is in a rough patch or needs rest, for example, in the case of a second game within 24 hours, the backup is called upon to tend the net.
Indeed, there is a goaltending change every game. Jake Allen performed well against the Vancouver Canucks, but Samuel Montembeault also had a solid game against the Bruins the night before. If one of them were playing poorly, the decision would be easier for the coach.
However, both deserve to play, and one could even add that Primeau deserves to play, having delivered a solid performance against the Detroit Red Wings.
Of course, number 34 was in goal against the Canucks on Sunday, so Montembeault will play tonight. This alternating system is working for now, but sooner or later, Martin will have to name a primary goaltender.