With less than a week to go before the Hoops face Barcelona in the second Group H Champions League tie, James Forrest has suffered a relapse in his battle to recover from a respiratory virus.
The winger was fighting his way back from the sciatic nerve problem which forced him to withdraw from Scotland's game against Macedonia when the virus first hit.
The 22-year-old has not played for Celtic since August 28 when he scored the third goal against Shakhter Karagandy to book Celtic's place in the Champions League group stage and with it a cheque for at least £15million.
Having had to pull out of last week's trip to the San Siro after being hospitalised by the effects of the virus, Neil Lennon hoped the youngster would be able to report to Lennoxtown at the start of this week to undergo further tests.
However, the manager has revealed that Forrest failed to make that appointment.
Lennon explained: "James didn't come in on Monday because his condition got worse again over the weekend.
"I don't know where we are with him at the moment."
The good news is Forrest did not have to be re-admitted to hospital.
But serious doubts still remain about when he will be fit to resume training.
Lennon said: "We are hoping he will be feeling a little bit perkier over the next couple of days."
Forrest isn't the only player facing a race to be ready for Tuesday's return visit to Glasgow of Lionel Messi and Co.
Joe Ledley and Beram Kayal have both been absent for the past few weeks with groin injuries.
The Welshman returned from international duty a fortnight ago with the problem, but managed to start against Hearts on September 14.
The 26-year-old had to be subbed in that match and has been receiving treatment since then.
Ledley has previously spoken about the possibility of eventually having surgery to rectify a hip issue he has been carrying for over a year.
But Lennon insisted his current incapacity has nothing to do with that particular condition.
The manager said: "It's not the hip, it's the groin that's keeping Joe out just now."
Lennon will get a full update on all his injured players before sitting down to consider the team he wants to face Kilmarnock on Saturday.
Even though Barcelona come to Parkhead just three days later, he will look to field his strongest available side at Rugby Park as the Hoops strive to gain the boost of a league win before they return to Champions League action.
"Emilio Izaguirre and Georgios Samaras were both rested for the Morton cup tie," said Lennon, who is expected to reinstate both experienced players at the weekend.
Kris Commons, Anthony Stokes and Adam Matthews, who came on as second-half subs against Morton, are also in line to return to the starting XI along with Fraser Forster.