Belisle has held opponents to a .182 batting average with four strikeouts in 6 1/3 innings. Belisle had familiarity with Baker, who was the manager in Cincinnati when Belisle made six starts for the Reds in 2008.
Baker also considered himself impressed from afar with Belisle's success pitching in Colorado, where he spent six seasons from 2009-14 with the Rockies and accumulated a 3.88 ERA.
"I've known him from before and I've faced him many times," Baker said. "He looked like the same Matt Belisle. … He got to Colorado and he settled on being a reliever, and he settled on being a very good reliever. Like I said, if you can pitch certain places, none more of a test to that than Colorado."
• Nats have bullpen decisions to make | Turner among roster moves
The Nationals signed Belisle to a Minor League deal with a non-roster invite on the day pitchers and catchers reported to camp, but Belisle owned an opt-out clause in that contract that he could have exercised Sunday. While Baker said the details of his contract still need to be figured out, he was emphatic about his desire to have Belisle.
"We want him on this team," Baker said.
Belisle joins a Nationals bullpen that will almost certainly be made up of Jonathan Papelbon, Felipe Rivero, Shawn Kelley, Oliver Perez, Yusmeiro Petit and at least one of three remaining relievers in camp: right-handers Blake Treinen, Trevor Gott and left-hander Sean Burnett.
Baker has said he is intrigued by the idea of carrying three southpaws in the bullpen, which bodes well for Burnett, another non-roster invitee who has not allowed a run all spring. Baker has spoken highly of Treinen and Gott, both hard throwing righties, but both players also have Minor League options.
The Nationals do not need a fifth starter until April 13, so it is possible they choose to option either right-handers Joe Ross or Tanner Roark on April 3, when rosters have to be set, in order to keep an extra reliever and extend their time to make a decision.
"I wish we could carry 27, 28 guys, but we just can't," Baker said.