Campbell, 27, got lots of playing time in Spring Training and traveled with the team to Montreal before being demoted to Triple-A.
He's hitting .355 with a .442 OBP, three HR and 24 RBIs in 33 games in Triple-A. He plays mostly first and third base, though he has also seen time at second base and shortstop this season.
The Mets did not announce a corresponding roster move. However, given Campbell's position, Josh Satin is the leading candidate to be demoted, according to multiple reports.
Satin went 0-for-5 on Friday, starting in place of Lucas Duda, who was unavailable due to food poisoning. Satin left five runners in scoring position. He has just three hits in 28 at bats this season.