The Argentina midfielder was keen on representing his country in Japan, and could also have featured for his country in the Copa America, but both competitions have now been put on hold.
Mac Allister was a key figure for Argentina during the recent Olympic qualifiers and those displays perhaps prompted Brighton to cut short his season-long loan at Boca Juniors.
Mac Allister made his Albion debut when he came on for the last 13 minutes during the 0-0 draw at Wolves on March 7. That proved to be Albion's last match before the Premier League was suspended.
International Olympic Committee member Dick Pound confirmed a delay had been agreed by IOC members. The Games will likely be rearranged for the summer of 2021.
The Japanese prime minister Shinzo Abe suggested pushing the Olympics back 12 months and could now take place between July and August next year.
The delay would mean Mac Allister will be available for Albion head coach Graham Potter throughout the summer.
Premier League football is suspended until April 30 at the earliest and Uefa want all domestic leagues completed by June 30.
The next meeting of all 20 top flight clubs is set for April 3, where they hope to plan a way forward.