Playing without star center Joel Embiid, who missed the game with a sore ankle, the Sixers allowed Atlanta to rack up 62 points in the paint while outrebounding Philadelphia 44-30.
"Definitely," Simmons said when asked whether physicality was the Sixers' biggest problem. "I think we were too soft."
In November, Simmons also used "soft" to describe his team -- and Embiid agreed -- after a 25-point loss to the Brooklyn Nets, though that was before the Sixers completed a major trade to acquire swingman Jimmy Butler from the Minnesota Timberwolves.
Butler would not go as far as Simmons did Friday night and took issue with his teammate's choice of words.
"I don't like the word 'soft,'" Butler said, and added a whistle. "I just don't think that we did what we were supposed to do. I'm not gonna say that anybody's soft. I just think that we got beat in every aspect of the game. They came out from the jump and did what they wanted to do, and they did that for 48 straight minutes."
With the game tied in the final minute, Butler lost the ball on a drive down the lane that led to Atlanta's John Collins finding separation and burying a fadeaway jumper with 25 seconds left for the lead. Butler had a chance at atonement but missed two free throws with 2.4 seconds left.
"I hope we're still together," Butler said. "Nobody pointing fingers at anybody."
Simmons, who had 23 points, 15 assists and 10 rebounds, wasn't naming names, but said he wanted to see more physicality from the entire team.
"I just don't think we're physical enough. I think defensively, we're not taking it personal enough when guys score on us. It should be a pride thing when somebody scores; you should be frustrated every time," he said.
"I think that comes with age -- we have a lot of young guys -- but at the same time, I think we have to expect more from each other to get better and get to that next level. 'Cause I know once playoff time comes, it's another level."
Asked whether he had spoken with his teammates about the issue, Simmons said: "We know. We know we gotta be more physical.
"We have to take it as a challenge. You don't wanna come out on the floor and have guys go at you every game. You wanna make that a pride thing. You wanna come out and get stops and lock down your man. And that's everybody, including me."
The November trade of Butler to join fellow stars Simmons and Embiid hasn't gone as smoothly as the Sixers had hoped. Embiid at times has complained about his role in Philly's offense, and Butler recently challenged coach Brett Brown about his own role in the offense, ESPN reported earlier this month.