Trevor Siemian will start Monday night against the Cleveland Browns and any other games that Darnold will miss.
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After their home game on Monday night, the Jets visit the New England Patriots in Week 3 before a bye week in Week 4. They visit the Philadelphia Eagles in Week 5.
When a reporter pointed out that mono usually take several weeks from which to heal, Gase nodded.
"Oh, I'm aware," Gase said. "Good thing we've got the early bye week."
Darnold was sent home by the team Wednesday with what Gase said was "strep throat or something." Team doctors later told Gase that antibiotics weren't working as quickly as expected, leading to the diagnosis.
"He's going through some of these tests to kind of see where he's at with all this," Gase said. "I know he's out this week. Then, past that, I'll have more information as we go."
Gase said he, offensive coordinator Dowell Loggains and athletic trainer Dave Zuffelato went to Darnold's home to deliver the news to the quarterback and he took it "not well," according to the coach.
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"He wasn't real happy about it," Gase said. "All three of us went over there and he knew something was up when Dowell and myself showed up. I just wanted to make sure it was delivered right, and I figured I could be the bearer of bad news on this one."
The coach says Darnold's biggest concern is weight loss. Gase adds Darnold has already lost about 5 pounds.
"That was something that was alarming to him, that he lost weight already in the last couple of days," Gase said. "He can't be in the building right now, so he's like, 'I need to get meals over here, I need to make sure that I'm getting enough of what I need to keep my weight.' I mean, he went right into, all right, how do I maintain?"
Gase said Luke Falk will be promoted from the practice squad to serve as Siemian's backup. The Jets could also look to sign an experienced quarterback for depth.
(ESPN and the Associated Press contributed to this report)