"He did not reach out to management with any explanation for his absence," Van Wagenen said in a statement. "Our attempts to contact him have been unsuccessful."
Cespedes, 34, was the designated hitter for Saturday's 7-1 loss to the Braves, going 0-for-4 with two strikeouts. He has served as the Mets' designated hitter in eight games this season, going 5-for-31 (.161) with 15 strikeouts.
He was not listed in the original lineup for Sunday's game, which began at 1:10 p.m. ET.
A two-time All-Star, Cespedes returned to the lineup this season for the first time since July 20, 2018. He missed most of the past two years with a series of leg problems, getting surgery on both heels and then a broken ankle after a tumble at his Florida ranch in a supposed run-in with a wild boar.
Because he didn't begin the season on the injured list with a foot injury, Cespedes' salary rose from $2,222,222 prorated ($6 million before the schedule was shortened) to $4,074,074 ($11 million before the change).
Information from The Associated Press was used in this report.