Still, the rookie stepped to the line with the confidence of a veteran.
Simmons scored a career-high 32 points, making two free throws with 5.6 seconds left to complete Philadelphia's rally from five points down in the final minute, and the 76ers beat the Chicago Bulls 116-115.
"Stepping up to the line, I knew I was going to knock em down. You have to have that mentality," he said.
Joel Embiid had 30 points and 13 rebounds, and Simmons added 11 assists and seven rebounds as Philadelphia won its sixth straight and snapped a four-game road losing streak.
Bobby Portis had a career-high 38 points and Zach LaVine added 23 for the Bulls, who were 18 for 34 on 3-pointers.
"He's a guy that continues to grow and develop," Bulls coach Fred Hoiberg said. "It's been fun to see him shooting the ball without any hesitation."
Chicago had a 115-114 lead and the ball with 8 seconds to go before Embiid deflected an inbounds pass to create a turnover. Simmons came up with the ball near halfcourt and was fouled by Denzel Valentine. He made both free throws.
"First of all, we needed a steal," Simmons said. "I saw Joel get it and he kind of lobbed it to me."
Portis had a chance for a game winner, but his short attempt -- with Embiid closing from the weak side -- just missed.
The 76ers raced to a 25-7 lead midway through the first quarter before Portis got hot and Chicago started making 3s.
Philadelphia's lead was down to 33-29 at the end of the quarter and the Bulls busted out for 40 points in the second to take a 69-67 halftime lead. Chicago was 9 of 12 on 3-pointers in the quarter, including 4 for 5 by Portis.
The 76ers were up by five early in the fourth before Portis scored six straight points -- on a 3-pointer and a three-point play -- to put the Bulls on top 101-100. A few minutes later, Portis dunked over Embiid to stretch the lead to 111-106 with just over four minutes to play.
LaVine hit a 3-pointer to make it 115-110 with 1:02 left.
"We stole that win," Embiid said. "We didn't play well. Offensively, I thought, we were moving the ball when we needed, but defensively I thought we sucked."
76ers: C Embiid has nine straight double-doubles. ... Simmons has six triple-doubles this season, which ties him with Wilt Chamberlain (three times) and Billy Cunningham for most in a season in team history.
Bulls: To look at some younger players during the final 25 games, C Robin Lopez and G/F Justin Holiday were removed from the starting lineup. Both were inactive on Thursday as Cristiano Felicio and David Nwaba (21 points) started; Also, Cameron Payne -- who played in his first game on Thursday -- will replace Jerian Grant as the backup point guard.
"It really is hard," Hoiberg said. "It's hard for Robin, it's hard for Justin and it's hard for Jerian, who's probably making as big a sacrifice as anybody."
Philadelphia is on the cusp of a playoff appearance this season, but the organization found itself in the same position as the Bulls the previous few years, playing mostly young players with the unspoken hope of improving draft position.
"When I look in the rearview mirror, it's not as painful as people make it out to be," coach Brett Brown said. "Losing stinks and the pain of losing is real, but the things that went into growing young players was really enjoyable for me. The development of young guys, giving them opportunities, explaining life in the NBA, what it takes to stay in the NBA, the culture -- stuff that nobody sees."
76ers: Host Orlando on Saturday.
Bulls: Visit Minnesota Saturday night.
Embiid dives into the crowd twice
Joel Embiid can't seem to find his footing in Chicago, ending up in the seats twice.