Neither player was with the team for its home opener at Citi Field against the Arizona Diamondbacks.
Mets manager Buck Showalter, without identifying the coach, confirmed that a member of his staff tested positive for the coronavirus and was absent Friday.
The team did not say whether Nimmo and Canha tested positive. Showalter would not say whether either player is vaccinated against COVID-19, but did acknowledge the rules for returning to the roster under baseball's new labor agreement are different for players who are vaccinated versus those who aren't.
"I tested negative," Showalter said. "But we don't need me."
Canha missed out on his first home opener with the Mets after signing a $26.5 million, two-year contract as a free agent in November. He batted .381 (8 for 21) with four walks on New York's 5-2 road trip to begin the season.
Nimmo, who had homered in consecutive games, might have missed his final home opener with the Mets -- he can become a free agent next offseason. The sharp-eyed leadoff hitter was 8-for-24 (.333) with six runs and four walks on the road trip to Washington and Philadelphia.
"It's part of life in the 2020s, let alone baseball," Showalter said. "We all kind of depend on each other to do the right thing."
To replace them on the active roster, the Mets recalled outfielder Nick Plummer from Triple-A Syracuse and selected infielder Matt Reynolds from Syracuse.
"That's why you prepare for these things," Showalter said. "Got a couple other people standing by in case we need it."