Striker Paul Heffernan gave Hibs the lead in the 17th minute when he raced through to knock the ball past Fraser Forster.
Forrest then replaced Kris Commons, who came off with a hamstring injury which makes him doubtful for the Champions League clash with Ajax, and a moment of magic in the 75th minute by the Scotland winger deservedly levelled the scores at Easter Road.
Celtic are still looking for their first points after two Champions League games and against Ajax they will also have to cope without captain Scott Brown, who was handed a three-match ban after being sent off against Barcelona at Parkhead earlier in the month.
Craig Moore scored a spectacular goal seconds into his Premiership debut to start Motherwell's comeback as Hearts slipped further behind at the foot of the table after a 2-1 loss.
The 19-year-old fired the ball into the top corner from 22 yards within 30 seconds of coming off the bench midway through the second half at Fir Park.
The strike cancelled out Ryan Stevenson's 62nd-minute breakaway opener and Shaun Hutchinson headed home with eight minutes left to secure third-placed Motherwell's fourth consecutive home victory.
Hearts' defeat was compounded by victories for nearest rivals Kilmarnock and St Mirren, which left them 13 points adrift.
An 89th-minute goal from Steven Thompson settled a thrilling contest as St Mirren picked up their first win of the season with a 4-3 success against St Johnstone at St Mirren Park.
The game was finely poised at 1-1 at half-time, Steven MacLean having got the visitors back on level terms from the spot following Kenny McLean's opener.
Paul McGowan and another McLean penalty put the hosts ahead on two further occasions in the second period, and they could have had another but Jim Goodwin missed from the spot in between.
Nigel Hasselbaink and substitute Rory Fallon twice hauled St Johnstone back into the game and looked to have nicked a point before Thompson provided the killer blow a minute from time.
Goals from Kris Boyd and Jackson Irvine gave Kilmarnock boss Allan Johnston his first win since taking over at Rugby Park in the summer - a 2-0 success against Ross County.
Boyd prodded the Ayrshire men in front after 17 minutes before Irvine's second with 19 minutes left in the second period ended the club's 12-game run without victory.
Calvin Zola's close-range strike early in the second half was enough to see fourth-placed Aberdeen seal a 1-0 victory over Dundee United at Pittodrie.
The former Burton striker pounced on Niall McGinn's cross to turn home the winner in an entertaining game.