Rask, who hasn't played in the NHL since undergoing offseason labrum surgery on his hip, signed a professional tryout agreement with AHL Providence on Thursday.
However, the Providence Bruins' two games scheduled for this weekend against Lehigh Valley have been postponed because of COVID-19 issues involving the Phantoms. Providence is not scheduled to play again until Friday.
"To be honest, I'm not sure when he's going to play," Cassidy said after Boston's morning skate in advance of Saturday night's game at Tampa Bay. "So right now I can't give you the definitive plan. He's practicing and we'll try to get him in some game action wherever we can. He might have to start playing here and miss some assignments in Providence."
A decision is expected during the Bruins' seven-game homestand that starts Wednesday night against Montreal. Boston visits Washington on Monday night.
Rask, 34, has spent 14 seasons with Boston and is the franchise leader in wins. He has been working out at the Bruins' training facility for more than two months and eyeing the resumption of his career, which includes two All-Star selections and winning the 2013-14 Vezina Trophy and 2019-20 Jennings Trophy.
Rask's next game action will be his first since June when he allowed four goals in a Game 6 loss to the New York Islanders in Boston's second-round loss in the Stanley Cup Playoffs.