The 20-year-old Nigeria striker took his chance at the Etihad Stadium with an eighth-minute goal.
Aguero's four-game domestic ban for getting sent off in Saturday's 3-1 loss to Chelsea means Iheanacho can look forward to a run of games in manager Pep Guardiola's first XI.
The 20-year-old Nigeria striker took his chance at the Etihad Stadium with an eighth-minute goal that cancelled out Celtic's opener, ironically scored by 19-year-old City loanee Patrick Roberts.
"There is no pressure on Kelechi Iheanacho. He just has to do his job," said Guardiola.
"I am so happy for the young players. We played well."
City had already secured a place in the last 16 and were guaranteed to finish below Group C winners Barcelona, while fourth-place Celtic already knew they had missed out on a Europa League berth.
"We were absolutely brilliant tonight in everything we did," said Celtic manager Brendan Rodgers, whose side held City to a 3-3 draw in September.
"We have grown in belief the more we have worked together. We are disappointed not to have won at least one of the games against City."
Roberts's goal means City have recorded just one clean sheet in their last 16 matches, but after the trauma of the loss to Chelsea, in which Fernandinho also saw red, this was a welcome return to normality.
With nothing riding on the game, Guardiola gave some playing time to young defenders Pablo Maffeo and Tosin Adarabioyo, but it was another City youngster who was the first to show.
Roberts, loaned to Celtic by City in February, embarrassed his parent club's defence, slipping past Gael Clichy from the right and sticking a left-foot shot past goalkeeper Willy Caballero.
Celtic had conceded the lead three times in September's draw at Celtic Park and history was to repeat itself here as City levelled within four minutes.
Nolito slid a pass down the inside-left channel for Iheanacho and he showed composure to guide a firm shot past Craig Gordon.
The sense that Guardiola was treating the game as an experiment was emphasised by his casual dress -- skinny jeans and sneakers -- as well as by City's novel 3-5-2 shape.
The system featured Pablo Zabaleta in midfield and he almost created a second goal with a sweeping pass that Iheanacho put wide.
Gordon was also required to palm away an improvised back-heel from Iheanacho, with Nolito's follow-up blocked.
But City looked patchy at the back and Moussa Dembele, scorer of a brace at Celtic Park, twice got in behind in the early stages.
Freed in turn by a sloppy Fernando back-pass and a neat James Forrest through ball, he was thwarted both times by Caballero, blazing wastefully wide of an open goal on the second occasion.
Despite losing Forrest to injury, Celtic made a bright start to the second half, with his replacement, Gary Mackay-Steven, curling a shot into Caballero's gloves.
Celtic threatened twice as the game approached its latter stages, Leigh Griffiths shooting wide and Caballero thwarting Mackay-Steven.
Leroy Sane curled a free-kick narrowly wide at the other end and might have won City a late penalty with a cross that struck Jozo Simunovic's hand, but Slovenian referee Slavko Vincic shook his head.
City were denied three penalties against Chelsea and Guardiola said: "Maybe one day we will get a penalty. Ask the referee. Every game it is like this."