Morris, 22, replaces Dillon Gee, who was designated for assignment after allowing eight runs in 3 2/3 innings in Sunday's spot start.
Not only did Gee's short outing force the Mets to use five relievers Sunday, it made Gee unavailable for Monday as well. Promoting Morris gives New York a fresh arm at a time when many in the bullpen are not.
"This is consistent with our desire to put the strongest possible 25-man roster together each night," Mets general manager Sandy Alderson said. "How long he'll be here will be a function of his performance but also of our need. One of the reasons for making this decision today was because Dillon would not be available out of the 'pen today because he pitched yesterday."
Drafted in the 10th round in 2010 out of Charlotte Amalie HS in St. Thomas, Morris becomes the 12th player born in the Virgin Islands to appear in the Majors (original Mets player Joe Christopher is another).
"My excitement level is through the roof," Morris said. "That's the dream. It can't get higher than this. I was just ecstatic."
New York's No.19 prospect possesses a 93-95 mph fastball and a plus changeup, but he struggles with command. He became a full-time reliever at Class-A Savannah in 2014 and pitched to a 1.69 ERA and converted 13 saves in 24 appearances at St. Lucie this season.
Morris had a 0.63 ERA in 41 appearances for Savannah in 2014.
The Mets had placed him on the 40-man roster to protect him from the Rule 5 Draft.
"He has a good arm. He has two swing and miss pitches in his repertoire," Mets manager Terry Collins said. "He's a young guy. There is a reason he's in A ball. He has to learn how to command his stuff. I told him this is a game, up here, where if you get strike one you can get outs. So it's important he can come up here and not be intimidated by anything. His stuff is good enough to get outs."
Collins added: "I think he's one of the great stories going."
Joe Trezza is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.