The winger sat out most of last season with an ankle injury and was badly missed before returning late in the campaign and making a brief appearance for Scotland at Euro 2020.
But he is absent from Celtic s training camp in Wales as new boss Ange Postecoglou bids to imprint his ideas on the squad.
Celtic confirmed via the club’s Twitter account, that Forrest was not present at the camp as he had been identified as a close contact of a positive Covid-19 case.
Christopher Jullien is also still in Glasgow as he works on his fitness following his recovery from a long-term knee injury sustained last year.
Celtic face Sheffield Wednesday in a behind-closed-doors friendly in Wales on Wednesday in preparation for the first leg of their Champions League second qualifying round against Danish League runners-up FC Midtjylland.
After the Owls at Dragon Park in Newport, they then play Charlton at the same venue on Saturday.
The cinch SPFL Premiership side then travel to Bristol City on Wednesday, July 14 before hosting Preston on Saturday week with a friendly match with West Ham at Parkhead also to come before the league campaign kicks off.
The club has recently been linked with a move for Swedish international defender Carl Starfelt of Ruben Kazan and are also reportedly monitoring Hibs striker Kevin Nisbet after recruiting Liam Shaw and Osaze Urhoghide from relegated skybet Championship side Sheffield Wednesday this summer.