Brendan Rodgers’ men slipped up at Parkhead on Saturday and could only draw 1-1 with Motherwell in their Scottish Premiership encounter.
Celtic’s Luis Palma had a penalty saved in the match, but the Bhoys looked set for victory after David Turnbull converted from the spot with just four minutes left.
However, Motherwell substitute Jon Obika levelled in the 90th minute and ensured a share of the spoils for the visitors and two dropped points for Celtic.
Ex-Scotland man Stewart believes that Celtic paid the price for being without Reo Hatate, Daizen Maeda and Micky Johnston.
He stressed that Celtic lack depth and Oh Hyeon-gyu is not able to offer the same quality that Giakoumakis did, with the attacker not taking his chances and impressing.
“There’s no question Celtic should have won the game, but they didn’t”, Stewart said on BBC Sportsound.
“They don’t have that same strength in depth as they did. They were without Hatate, Maeda and Johnston today.
“Oh Hyeon-gyu is the fall back guy when they’re looking at something different or Kyogo isn’t firing on all cylinders.
“He’s had a lot of chances and he doesn’t score enough goals.
“The difference between him and Giakoumakis is staring you in the face.”
Giakoumakis left Celtic for MLS side Atlanta United, where he has been prolific for the American side.
Whether Celtic will enter the January transfer window to try to sign another striker remains to be seen.