Hoops manager Brendan Rodgers confirmed there is "big interest" in the French under-21 international, who has scored 51 goals in 94 appearances since joining Celtic in 2016.
Dembele was at Celtic Park with his representatives to discuss his future while his team-mates trained in preparation for the second leg of their Europa League tie against Suduva on Thursday which stands at 1-1 from the first game in Lithuania last week.
"Moussa and his representatives were speaking to the club today," Rodgers told reporters at his pre-match press conference.
"He's not a player we want to lose. He's a phenomenal talent. He's not going to be here forever. Like a lot of the European boys, they develop at a great club and look to move on."
Rodgers has openly voiced his frustration at Celtic's inability to add quality this summer to a squad that has won back-to-back domestic trebles in the two seasons since the former Liverpool manager took charge.
Celtic were knocked out of the Champions League third qualifying round 3-2 on aggregate by AEK Athens and face Rangers in Steven Gerrard's first taste of the Glasgow derby at Celtic Park on Sunday.
"We don't want to lose a top class player without having a replacement," added Rodgers.
"(There's) not been an official bid, but interest. It's the 'what-if' scenario. You always have to be prepared for that. That's natural. It’s not an ideal situation with a few days (of the transfer window) to go."
Lyon are in the market for a striker with Real Madrid set to activate a buy-back clause for Mariano Diaz, a year after he left the European champions.