Celtic lost Emilio Izaguirre broke his leg before a late goal from Anthony Stokes gave them a 1-0 victory over Aberdeen.
The Honduran left-back was carried off on a stretcher after going down in agony following a sliding challenge from Peter Pawlett.
Boss Neil Lennon revealed tests had shown up a fracture with the club soon confirming Izaguirre had broken his fibula.
Further tests will reveal whether Izaguirre requires surgery on his right leg.
The injury happened when the 25-year-old fell awkwardly following the challenge early in the game, which Celtic won 1-0 courtesy of a 74th-minute goal from Stokes.
Lennon said: "It's a huge blow for us to lose him and we don't know how long he is going to be out for.
"He is being assessed at the minute and we'll know whether he requires surgery and, if he does, he will be out for three months anyway.''
Lennon held his head in his hands when he was told the initial diagnosis that Izaguirre had suffered a suspected fracture.
"You can always tell by a player's reaction and Emilio is not a player to feign injury,'' Lennon said.
"So the concern was immediate and he was taken into the dressing room and looked at and the word came out it was a fracture.''
Lennon was already missing defenders Thomas Rogne, Glenn Loovens and Mark Wilson through injury, although the latter is due back from a groin injury for next week's SPL clash with Dundee United.
Celtic ended up with Adam Matthews and Charlie Mulgrew in the full-back positions and Lennon may look to bring in reinforcements.
"We might have to look for another left-back,'' Lennon said.
Celtic are currently in the transfer market for a goalkeeper with a deal for Poland international Grzegorz Sandomierski coming to fruition.
Celtic have been in negotiations with the 21-year-old's club, Jagiellonia Bialystok.
"I'm not sure if it's a done deal, I haven't spoken to Peter today, but we might be further down the line with that one,'' Lennon said. "He's a goalkeeper we do like and we feel he has huge potential.''
Kris Commons created the only goal of the game dispossessing Richard Foster before setting up Stokes in the 74th minute.
The win took Celtic top - but Motherwell later deposed them as Jamie Murphy gave them a 1-0 win over 10-man Hearts.
Four minutes after the interval Jambos midfielder Ian Black was shown a straight red card by referee Stevie O'Reilly for a challenge on Keith Lasley.
And the Steelmen capitalised on their advantage on the hour mark when Murphy converted a pass from Nicky Law before Lasley, already booked, was sent off in the 86th minute after picking up his second yellow card.
