Manager Jerry Manuel confirmed the move late Sunday morning.
"That's just where we are," Manuel said. "That's our best option. You don't know what you're going to get, that's true. But you have talent."
Niese, 21, excelled with both Double-A Binghamton and Triple-A New Orleans this year, finishing the latter assignment with a 5-1 record and 3.40 ERA. And although he will make his Major League debut against the Brewers, one of the league's best offensive teams, the Mets insist that Niese has the makeup to excel.
"Mentally, he's prepared," said Nick Evans, a teammate for much of the early season at Binghamton. "I don't have any reason to doubt that he's going to be ready to go."
"We think very highly of Jonathon Niese," said Tony Bernazard, the Mets' vice president of player development. "He's a 21-year-old that we expect to be an important member of this organization, this Major League team. You have to perform at some point. You have to perform, and it happens to be Tuesday against a very good team."
Niese's successes, quite simply, depend upon his curveball, a looping pitch in the mold of Barry Zito. Also equipped with a fastball and changeup, Niese sports average velocity, but a knowledge of how to pitch beyond his limitations.
He made three starts for the Mets during Spring Training -- all of them after a tutorial with Hall of Famer Sandy Koufax -- and finished with a 3.12 ERA. And although his rank as one of the team's top pitching prospects came by default, after the Mets traded away multiple starters to acquire Johan Santana last winter, Niese has done nothing this season but prove he deserves that tag. Quite simply, he has excelled at every level.
And that has earned him a date with the Brewers in a game with playoff implications.
"I don't know if you can ever prepare a young man for that," Manuel said. "You can talk about it, but that is one of those things that you have to experience."
Several other Minor Leaguers -- not including outfielder Fernando Martinez, the Mets' top overall prospect -- will join Niese in Milwaukee on Monday, when rosters officially expand to 40. Pitchers Robert Parnell, Ricardo Rincon, Carlos Muniz and Al Reyes -- recently signed as insurance for the back end of the bullpen -- will earn the call from Triple-A. So too will Argenis Reyes, who became expendable earlier this week when Luis Castillo returned from the disabled list, and fellow infielder Ramon Martinez. Outfielder Marlon Anderson will also come off the disabled list in time for Monday's game.
The decision to leave Fernando Martinez behind stemmed from the current makeup of the Mets' roster. With Ryan Church back from the disabled list and Daniel Murphy bullying his way into consistent playing time, Martinez, 19, would have provided little more than clutter.
"We don't have a need to bring up an extra outfielder right now," Bernazard said.
Anthony DiComo is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.