Butler, who started in Miami's 125-105 win over the Denver Nuggets on Saturday, remains inside the NBA's bubble in Orlando. Heat head coach Erik Spoelstra declined to elaborate on the reason for Butler's absence.
Teammate Jae Crowder hinted at the All-Star possibly being in self-isolation.
"We're going to talk to him as soon as he gets out of quarantine and whatever he's in," Crowder said.
"It's just next-man-up mentality, from the standpoint of staying locked in and engaged, because we know he's locked in and engaged once he's able to get back with us," he added. "It's definitely a curveball for all of us to hear stuff like what's going on with him, and you never know what to expect."
It's unclear whether Butler's absence is related to a positive COVID-19 test. The NBA's testing protocols have been successful so far, as no players have tested positive over the past two weeks.
Miami (42-24) is set to face off against the Toronto Raptors (47-18) on Monday in the second of its eight seeding games. Butler wasn't listed on Sunday's injury report and is expected to be available, sources told Marc Stein of the New York Times.