Neil Lennon’s men lost a third straight match in all competitions after being beaten 4-2 by AC Milan on Thursday evening, and have now conceded 27 goals in their last 11 outings.
The Hoops have alternated between starting Scott Bain and summer signing Vasilis Barkas in recent weeks, with the Greek given the nod at the San Siro last night.
Neither player has been able to stop the rot in the goals against column, however, and after a high-profile transfer saga that ultimately saw the Bhoys priced out of a permanent move for Southampton stopper Forster, Stewart has claimed that the Glaswegian giants could be willing to revisit negotiations in the coming weeks.
The 32-year-old was an integral member of Celtic’s title-winning side last season, but has fallen a long way down the pecking order for the Saints this term.
Writing on his personal Twitter account, Stewart said: “Wouldn’t surprise me if Celtic are already trying desperately to get Forster back on some sort of deal.
"Goalkeeping position is desperate just now.
"While the defence simply cannot continue to making the mistakes they are if they’ve any hope of resurrecting their season.”
In a subsequent tweet, Stewart added: “It takes a long time and lots of hard work to develop and put the fear factor in opponents. Celtic have lost that now and it’s not going to be a quick fix to get it back.”