The team didn't offer an explanation, but general manager Don Sweeney said shortly after the announcement on Friday that it was not health related. He also asked for privacy on behalf of Rask.
Rask has started only seven of the Bruins' 15 games this season and acknowledged having been outplayed recently by Jaroslav Halak.
Rask came on in relief in the Bruins' 8-5 loss to the Vancouver Canucks on Thursday night. Halak was pulled after allowing five goals on 19 shots. Rask allowed three goals on the first eight shots he faced.
Boston, which hosts Toronto on Saturday, has dropped three of its past five games.