According to Luca Bargellini of Italian outlet Tuttomercatoweb (h/t the Daily Star's Jamie Styles), United scouted the midfielder earlier in the season, but Arsenal will "try their luck" by making an offer for him, though he reportedly has a £55 million buyout clause.
Mario has attracted widespread attention from Europe's elite, with Juventus, Paris Saint-Germain and Atletico Madrid also named as interested parties.
Sporting youth coach Tiago Capaz has given the 23-year-old rave reviews, according to Styles: "I have no idea what his limit is. Week by week he gets better and better.