The 21-year-old Australian, who came close to averaging a triple-double in the Sixers' first-round series victory over the Miami Heat, failed to score a basket on Thursday as the 76ers squandered a 22-point lead and fell into an 0-2 hole in their best-of-seven second-round playoff series against the Celtics.
"It's just about the worst I've played all year," said Simmons, who finished with one point, five rebounds, seven assists and five turnovers.
The Celtics threw a number of different defensive looks at Simmons, but he said the problem was "self-inflicted" rather than the result of Boston's efforts.
Simmons said he was "overthinking" in the game.
"Thinking too much about everything, about every play on the floor. I think I'm looking at it in too much detail and not just playing," he said Friday at the 76ers' off-day practice.
Simmons spent much of the second half on the bench, but he said his morale remains high.
"I'm a very confident person every time I step on the court, that's not going to change," he said.
Simmons said he'd received "words of encouragement" from Sixers legend Allen Iverson, who reached out for a phone pep talk.
"He just said play the game I know how to play," Simmons said. "And that's just second nature to me."
Simmons has received support from less expected quarters as well.
"The biggest thing that people don't understand is that every player has that night," Mitchell said ahead of the Jazz Western Conference game against the Houston Rockets.
"It just so happens that it happened to (Simmons), and I expect him to respond back. He's a good player. Good players respond back, and it's all about the response."