Celtic legend Davie Hay has urged the Bhoys to make a quick decision on left-back Boli Bolingoli and sign a replacement for the defender if he has been judged as not up to the mark.
The Scottish champions snapped the left-back up from Austrian side Rapid Vienna, paying a fee of £3m to secure his services.
However, Bhoys boss Neil Lennon left Bolingoli on the bench against Cluj in the Champions League in midweek, raising eyebrows by playing Callum McGregor at left-back instead.
There are now question marks over whether the 24-year-old has failed to win over Lennon and his backroom staff in training, and Hay thinks with the clock ticking on the summer transfer window, Celtic must make a decision on Bolingoli.
"As I have mentioned elsewhere, this domestic season is going to be a huge test", Hay wrote in his Evening Times column.
"Rangers have improved which means Celtic must get better as well.
"The club have a strategy and I’ve no problem with that.
"However, they also need to win back some trust which is why Neil will be given some funds.
"It might be we see a new left-back because Boli Bolingoli wasn’t trusted and if the summer signing isn’t good enough, that has to be addressed", he added.
Celtic will take a big financial hit through not reaching the Champions League group stage and will now need to play a qualifier to reach the Europa League group stage.
The transfer window in Scotland, along with much of the rest of Europe, closes on 2nd September.