Iheanacho, who came on as a substitute, scored the winning penalty for the Bhoys.
Tounekti, starting on the left wing, was a constant threat despite being at fault for Kilmarnock’s equaliser.
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Sutton, speaking after the match on X, said: “What a start to his Celtic career by Kelechi Iheanacho.
“Cool penalty to win it in injury time.
“Tounekti excellent and begs the question what if he’d have been signed pre-Champions League.”
He also acknowledged Kilmarnock’s “brave display” and praised Watson’s “stunning header” that brought the scoreline level.
Celtic’s victory sees them top the Scottish Premiership, nine points clear of Rangers.
Their next fixture is in the Premier Sports Cup, where they will face Partick Thistle.
The match is a week today.