Manager Joe Girardi told reporters that Realmuto has not tested positive for COVID-19.
"All his tests are fine. You can't let someone with a stomachache and a small fever walk around your clubhouse today," Girardi told reporters Thursday. "In a normal year, you wouldn't worry about it, but you can't do that now."
The Phillies recalled catcher Rafael Marchan from Triple-A Lehigh Valley.
Realmuto, who signed a five-year deal with Philadelphia this offseason that has the highest average annual value ($23.1 million) for a catcher in MLB history, is hitting .314 with four home runs and 16 RBIs this season.
The two-time All-Star selection did not play in Wednesday's game against the Nationals after taking a foul ball off his left knee in the seventh inning of Tuesday's game.
A player goes on the COVID-19 IL by testing positive himself or for contract-tracing purposes after possibly being exposed to someone who tested positive.
Marchan made his major league debut last season, hitting .500 (4-for-8) with a homer and three RBI. The 22-year-old is hitting .250 in six games at Lehigh Valley this season.
The Associated Press contributed to this report.