Mane, who downed Manchester City with a hat-trick on Sunday, is said to be a target for both the Citizens and Manchester United.
But the Saints, as ever, appear to have a plan B in place in the shape of Mertens.
The Belgian has started six times for Napoli in Serie A this season, but has still managed to score five goals - including a hat-trick in April's 6-0 hammering of Bologna.
Liverpool were previously linked with Mertens, however any interested parties may have to act fast after Italian reports this week claimed that the soon-to-be 29-year-old is closing in on a new contract, which would tie him to Napoli until 2020.
Mertens will be familiar to Southampton head coach Ronald Koeman, having played against his Feyenoord team while on the books of PSV Eindhoven.
Koeman has previously utilised his knowledge of the Dutch game to bring Dusan Tadic, Graziano Pelle, Jordy Clasie, Cuco Martina and Virgil van Dijk to St. Mary's.