Forrest netted with a brilliant counter-attacking goal in the 17th minute, and Celtic responded immediately thanks to some impressive wing play from Johnston after Sebastian Larsson had levelled from the spot in the 33rd minute.

However, Lennon was left sweating on the fitness of Ajer with the first derby of the season against Rangers looming, while Odsonne Edouard left the field with cramp.

Ajer, who has been Celtic’s best defender this season in an inconsistent defensive selection, went off before the opening goal after struggling with knee and shoulder injuries in recent weeks.

Celtic were already missing right-back Hatem Abd Elhamed and centre-back Jozo Simunovic through injury, although both could return for Sunday’s game.

Anthony Ralston came on for Ajer, with new right-back Moritz Bauer not registered for the game following his loan move from Stoke.

Celtic’s night could have been more difficult had Kolbeinn Sigthorsson’s 18-yard volley on the turn been inches to the right instead of bouncing off the base of Craig Gordon’s post.

Forrest got the first goal after playing an excellent one-two with Edouard, beating a defender and finishing into the top corner with his left foot.

Nabil Bahoui failed to make the most of two decent chances, the second after Jullien dived in on the edge of the box, but the hosts got a lifeline when Boli Bolingoli accidentally ran into Larsson inside the box. The referee pointed to the spot and Larsson sent former Sunderland team-mate Gordon the wrong way.

Celtic responded immediately. Johnston tormented full-back Daniel Granli and Larsson on the left wing and his low cross was bundled over the line by goalkeeper Oscar Linner.

Celtic threatened again before the break and Edouard dinked a shot off the post after a brilliant return pass from Ryan Christie, before Linner got a strong hand to push the latter’s 30-yard strike over the bar.

AIK had an early chance after half-time when Chinedu Obasi waltzed around Jullien, but Gordon and Nir Bitton were alert to close him down and the shot was blocked.

Celtic went on to control the game and Forrest and Vakoun Bayo had strikes saved before they added to their lead.

Jullien slid in front of Scott Brown to force Christie’s low cross over the line following a short corner four minutes from time and substitute Morgan headed Forrest’s cross home in stoppage time.