Monreal is a target for Real Sociedad this summer and the Gunners have identified Firpo as a replacement, says the Daily Star.
Although something of a utility man, Firpo is a left-back first and foremost.
The 21-year-old can also play in central defence as well as on the left flank.
The defender made his Betis debut just five months ago but was a star performer for Quique Setien's team in the second-half of last season.
He grabbed two goals and two assists in just 14 games in the Spanish top-flight.
Junior's contract does not expire until 2021, although it contains a £22m buyout clause.