The Polish striker joined Neil Lennon’s side from Jagiellonia Bialystok for a fee of around £3.6 million in January, and has since scored three goals in 20 outings for the Glaswegian giants.
For large stints this season, the 22-year-old has been forced to play second fiddle to Odsonne Edouard and summer signing Albian Ajeti, but was given a brief cameo in Thursday evening’s 4-2 Europa League defeat to AC Milan.
Speaking after the match, however, Stewart admitted that Klimala’s form since arriving in Glasgow has been so underwhelming that he was never convinced the striker could make a meaningful difference for Lennon’s ailing side.
"Without being overly critical of Patryk Klimala, he doesn’t look good enough”, he told BBC Sportsound.
"This is where another aspect of the decision-making within the club doesn’t seem to have been working particularly well at the moment.
"Klimala was bought, a good few million spent on him, and I’ve never seen any part of Patryk Klimala that, to me, has thought, ‘Oh yeah, I can see what they were buying there’.
"You ask yourself the same question about a number of other players that simply haven’t cut the mustard so far.
"They recruited down the spine of their team, which I think was sensible to do, but so far, the players they've brought in haven’t [performed].”
Klimala’s last goal for Celtic came in the 2-0 win over St. Johnstone at the beginning of October.