Forrest was hurt in the first half.
The gap between Rangers and Celtic remained at four points as Tom Rogic's strike earned a narrow home win over Motherwell.
The Australian steered in a well-crafted set piece move after the hosts had to weather a first-half storm of pressure from Graham Alexander's men.
Celtic took the lead in the second half but had to hold on for dear life.
Motherwell is fifth on goal difference.
James Forrest went off in the first half with an injury, and it's not the only one for Ange Postecoglou.
Reaction and how it happened.
The pre-match build up was about how Postecoglou would conquer Celtic's striking dilemma.
The Celtic manager was without a glaring alternative because of the absence of Jota, Albian Ajeti, and Giorgos Giakoumakis.
At first, David Turnbull was the card Postecoglou would play. The false nine started as a false nine, but was replaced by Forrest, Abada and Rogic.
Forrest didn't stay in that group for long, as he succumbed to injury early on.
Motherwell almost made the hosts pay in the first-half, as they took a while to find their rhythm in this new formation.
Tony Watt had a penalty claim waved away by the referee, while Kevin van Veen had a penalty claim waved away.
Motherwell's defence failed to react to the cutback and Celtic scored a crushing blow when Rogic swept home a set-piece move.
The timing of the free-kick just before the break had an impact on Alexander's team.
Celtic emerged from the tunnel with renewed energy and determination to find a second.
Liam Kelly stopped the ball when it was delicately threaded by Turnbull, but he missed the chance to make it two.
Abada was flagged for being in an area where he was not supposed to be, but he hit a free head off the bar. The trio continued to menace, with all three testing the Motherwell keeper throughout the second-half, but the lead would remain at just one goal.
The man of the match was Tom Rogic.
A moment of magic was needed to break the stalemate.
What did we learn?
The pre-match response from Postecoglou seemed to hint at Celtic's strikersless formation. He didn't claim it was a 3-6.
Motherwell had three wins from their last five and that was a perfect cocktail for a scare at Celtic Park.
Graham Alexander's men couldn't change the usual narrative because Celtic have only conceded two at home in the league this season.
The Steelmen ran out of steam at the end of the first half and were deflated by the timing of the goal.
What they said.
"We had opportunities to make it more comfortable," said Postecoglou. You always give the opposition a chance to hang in when you don't take those opportunities.
We handled it well. It was a big victory for us in the middle of a busy schedule.
I've had challenges since I walked through the door. There's no need to cry or moan about it.
Motherwell boss Graham Alexander thought it was a cheap decision. If you give a free-kick for every challenge, it would stop-start.
I'm proud of my players. We gave it everything.
What are you going to do next?
The League Cup final is on Sunday and Celtic will face a long trip to Dingwall on Wednesday.
Motherwell will play their final game before Christmas next Saturday.
Mcgregor was 42.
Substituted for Johnston at 18 minutes and Ralston at 78 minutes.
Johnston is a person.
Substituted forDonnell 73' minutes.
Substituted for Robertsat 87' minutes.
Substituted forShields at 73' minutes.
Celtic defeated Motherwell 0 to 0.
Celtic leads Motherwell by one at the end of the second half.
The defensive half is where Anthony Ralston won the free kick.
Tony Watt had a foul.
An attempt was missed. Liel Abada's shot from outside the box is close, but misses to the right. David Turnbull assisted.
Motherwell. Tony Watt is caught off guard by Jordan Roberts' through ball.
Motherwell, substitution. Kaiyne Woolery was replaced by Jordan Roberts.
Motherwell. Tony Watt is caught off guard by Barry Maguire's through ball.
Foul by Nir Bitton.
Stephen O'Donnell won a free kick.
Carl Starfelt had a foul.
Tony Watt won a free kick.
An attempt was missed. The shot from the centre of the box is too high forConnor Shields. Tony Watt assisted.
The foul was committed by Cameron Carter-Vickers.
A free kick is won in the defensive half.
An attempt was missed. The left footed shot from the centre of the box misses to the left. Josip Juranovic assisted.
Celtic. Anthony Ralston is the new person.
An attempt was missed. The shot from outside the box is too high for Tomas Rogic. Michael Johnston assisted.
Corner in Motherwell. Carl Starfelt Conceded.
Corner in Motherwell. Carl Starfelt Conceded.