The team has not decided who will take Nieve's spot in the rotation. The likeliest candidates are righty Nelson Figueroa and southpaw Jon Niese.
Sullivan, a speedy, defensive-minded player signed during the winter, spent the past four seasons in the Rockies organization before batting .290 in 85 games with Triple-A Buffalo this year.
The team called up Sullivan rather than 22-year-old prospect Nick Evans (who played 15 games for the Mets prior to the All-Star break) because of Sullivan's versatility.
"In our situation now, in Cory's defense, he has a little bit more versatility than Nick," manager Jerry Manuel said. "He can play center field, he can play right field. Nick is a first baseman/left fielder basically, and he's still growing and learning those positions."
A regular in 2005 and 2006, the 30-year-old Sullivan played 18 games in the Majors last season and will be making his Mets debut when he sees game action for New York.
With Buffalo playing at Toledo on Sunday and traveling to Louisville for a game on Monday, Sullivan had not made it to Nationals Park before the game against Washington on Monday.
Though Sullivan may not be available on Monday, the Mets could use him in the near future as a way to shake up their recently anemic offense.
"If we run into a situation where we need a little bit more offense someplace out there, we'll start to play him," Manuel said. "But [Angel] Pagan has played very well in center field. Jeremy Reed is now getting a pretty good look, but just like anyone else, there has to be some production from there. And if not, that probably would get Sullivan an opportunity to play center."
Mark Selig is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.