Iheanacho strikes as City, Celtic draw
Aguero's four-game 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 with an eighth-minute goal that cancelled out Celtic's opener, ironically scored by 19-year-old City loanee Patrick Roberts.
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.
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 3-3 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.
Guardiola introduced Jesus Navas for Maffeo and the winger teed up Nolito to turn the ball up at the back post, only for an offside flag to curtail the Spain international's celebrations.
Celtic threatened twice as the game approached its latter stages, with 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 his side a late penalty with a cross that struck Jozo Simunovic's hand, but Slovenian referee Slavko Vincic shook his head.