Butler, who played for the 76ers last season, made 14 of 20 shots in the 137-106 win at American Airlines Arena in Miami.
"We were soft," Simmons told reporters. "It's tough to turn it around [on the road]. But that's our job. We have to come in ready."
The Heat (34-15) outscored the 76ers (31-20) 81-54 in the second half of the blowout win after leading by just four points at halftime. Bam Adebayo scored nine points to lead the Heat to a 30-26 edge in the first quarter and Butler poured in 17 points in the second quarter. The Heat led 56-52 at the break.
Butler netted another 15 points in the third frame, pushing his total to 38 points. He sat out the fourth quarter as the Heat held a 20-point edge entering the final frame.
Butler also grabbed seven rebounds, three steals and handed out two assists in the win. Goran Dragic scored 24 points and had six assists off the bench for Miami. Adebayo had 18 points, 11 assists and eight rebounds and Duncan Robinson had 19 points for the Heat. Derrick Jones Jr. chipped in 15 points.
"The physicality side of things, we gotta step that up," Simmons said. "That comes with experience, also just comes with personality.
"Don't get bullied. Fight over screens, get through screens. If you gotta knock somebody over, knock 'em over. If you gotta hit somebody in the face and knock 'em down so they don't score, hit 'em in the face."
"I thought that our defense was poor," 76ers coach Brett Brown said. "When you create three turnovers for a game, it's just not a reflection of who we are."
The Heat start a six-game road trip with a game against the Los Angeles Clippers at 10 p.m. EST Wednesday, and the 76ers finish their four-game road trip against the Milwaukee Bucks at 8 p.m. EST Thursday.