Dulay had been battling Achilles and hamstring injuries which restricted him to 6.6 points, 2.5 assists, and 1.2 steals per game in the current ASEAN Basketball League (ABL) campaign.
Dulay was in his second tour of duty with the Slingers, having signed with the team prior to the start of the 2012 ABL season.
In total, he has played 33 games for the Slingers franchise with career averages of 9.4 points, 3.6 assists per game, and 1.6 steals per game.
"Don is such a great person and we tried to stay with him as long as we could. Unfortunately, in such an important role as our starting point guard, we were struggling without Don being at 100%," Slingers coach Neo Beng Siang said.
"Don has been a great team player and a real professional in his time with the Slingers."
Slingers general manager Michael Johnson added: "We are sad to be parting company with Don who is a quality individual and has given so much to the club on and off the court."
"Don leaves Singapore knowing he will always be an important part of the Slingers family. He handled the news that the club is making the change in a true professional manner and with real class. We wish him the best as he heads back home to Manila with his wife Glaiza."
Former Saigon Heat guard Morrison won't have much time to settle into his new club as the Slingers travel to Kuala Lumpur on Friday for a crunch clash against the Westports Malaysia Dragons.