The 32-year-old was injured while reaching to apply a tag in the sixth inning of New York's 5-4 extra-inning victory over the Miami Marlins on Saturday.
The Mets got better news on outfielder Starling Marte, who left Saturday's game in the fourth inning with left groin tightness but apparently will avoid being placed on the IL.
Marte sat out Sunday's game against the Marlins and said through a translator that his MRI "came out fine, just a slight inflammation." He said he had been feeling tightness in his groin for a couple of days before coming out of Saturday's game.
Mets manager Buck Showalter said Sunday after the Mets' 2-0 loss to the Marlins in 10 innings that Marte is day-to-day and he hopes the outfielder can return to the lineup in the near future.
This is the second stint on the injured list this season for McCann, who required surgery for his broken left hamate and spent six weeks on the injured list. He was reinstated on June 24 and started eight games. New York manager Buck Showalter indicated that McCann was expected to be out "longer than 10 days."
Patrick Mazeika was recalled from Triple-A Syracuse in a corresponding move.
Tomas Nido, who took over for McCann on Saturday and delivered the tying hit and scored the winning run in the 10th inning of the 5-4 victory, was in the starting lineup Sunday.
Showalter also noted that the plan for injured outfielder Travis Jankowski (left fourth metacarpal fracture), who is currently on a minor league rehab assignment with Syracuse, is to have him join the team in Atlanta.
The Associated Press contributed to this report.