The 37-year-old Poland captain started the match and was substituted in the 63rd minute without scoring.
Finland international Robin Lod replaced him, Polish state news agency PAP reported.
It said Chicago took an early lead in the 18th minute after Inter Miami defender Ian Fray's back pass eluded goalkeeper Rocco Ríos Novo and rolled into the net for an own goal.
Inter Miami equalized nine minutes later when Uruguay striker Luis Suárez converted a penalty.
Suárez, captaining the team in the absence of Lionel Messi, who is resting after the World Cup, scored again in the 51st minute to give the hosts the lead.
South Africa's Puso Dithejane leveled the score for Chicago in the 67th minute, but Preston Plambeck netted the winner for Inter Miami three minutes from time.
After the match, Lewandowski said he was not yet ready to play a full 90 minutes.
"I still need a little more time to get back to full fitness," he told reporters. "For me, this first match was mainly about putting into practice the work I've done in training. I still need some time to regain freshness, but over the coming days and weeks that will improve."
He also said the hot and humid conditions had made his debut more demanding.
"It wasn't easy. The conditions were challenging, and the weather was very different from what we're used to," Lewandowski said, as quoted by the PAP news agency.
He added that, after only a handful of training sessions with his new teammates, it was unrealistic to expect the team to gel immediately.
"This was the first match. You don't build understanding overnight or even in a week. It will take some time to settle in, but every game will help us understand each other better, and my physical condition will keep improving."
Lewandowski said a draw would have been a fairer outcome.
"It's a pity we lost," he said. "At one stage the game was wide open, with chances for both teams, but we made some simple defensive mistakes. Still, every match will help us improve."
Chicago Fire are third in the Eastern Conference with 26 points, while Inter Miami moved into second on 34 points after playing one game more than Chicago.
Nashville SC lead the conference with 39 points.
Chicago's next match is away to New York City FC on Saturday.
The Polish striker joined Chicago on June 29 after four seasons with Barcelona.
Robert Lewandowski poses with a jersey after being introduced as a Chicago Fire player at a press conference on July 14, 2026. Photo: Jacek Boczarski/Anadolu/ABACAPRESS.COM via PAP
Lewandowski's debut had originally been scheduled for last Thursday, but Chicago's home match against the Vancouver Whitecaps was postponed because of hazardous air quality caused by wildfire smoke drifting south from Canada.
The match has been rescheduled for October 6.
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Source: IAR, PAP