Sources have told ESPN that Barca are struggling to make room for Messi to be registered in time for the start of the season due to financial restrictions.
However, Messi is expected to sign a new five-year deal at Camp Nou with his contract having expired last month.
Ronaldo's future at Juventus is also up in the air with the forward set to become a free agent next summer, and sources have told ESPN that his representatives are gauging interest from some of Europe's top clubs.
The possible Messi and Ronaldo showdown will not be the only attraction on the day as Barca Women will feature at the Gamper for the first time ever.
Ronaldo won't be short of suitors after winning the Euro 2020 Golden Boot this summer after scoring five goals and setting up one more in the competition.
The 34-year-old claimed his first major international title for his country and was named Player of the Tournament.
The two superstars have won 11 Ballon d'Or trophies between them and faced each other last season during the Champions League group stage.
Ronaldo, who missed his team's 2-0 defeat at home to Barca on Oct. 20 due to a positive COVID-19 test, scored twice from the penalty spot as Juventus won 3-0 at Camp Nou on Dec. 12.
The event will mark the return of football fans to Camp Nou, with a predicted attendance of 19,869, as the club is only permitted to have a maximum 20% capacity due to the pandemic.
The Gamper Trophy is an annual friendly match held in August before the start of Barca's campaign.
The last time Barca faced Juventus in the Gamper was in 2005 when an 18-year-old Messi produced a stunning display as the Italian giants won the trophy on penalties after a 2-2 draw.
During the match, then-Juventus boss Fabio Capello was so impressed that he attempted to sanction a loan move for Messi at the time but Barca coach Frank Rijkaard laughed and rejected the proposal.