The Mets selected Monell's contract before Tuesday night's game against Baltimore.
The 29-year-old Monell was hitting .397 with four home runs and 15 RBIs in 17 games at Las Vegas. He wasn't in the starting lineup against the Orioles.
Monell went 1 for 8 with San Francisco in 2013 in his only major league stint. The Mets signed him in the offseason.
Campbell hit .200 with one homer and five RBIs for the Mets. He took over at third base after David Wright was injured and got off to a nice start, but tailed off recently and went back to the bench.