Despite having only turned 18 in June, Mendes has already been provided with five starts and two substitute outings in Portugal's top flight.
Each of those appearances have come since the resumption, potentially an indication that Sporting view the teenager as an important player for next season.
However, Calciomercato claims that United and Liverpool are showing an interest in a player who has recently been deployed as a left wing-back.
Although United are well stocked for options in that area, Premier League champions Liverpool may view the youngster as the ideal backup to Andrew Robertson.
Juventus are also said to be active in the race for Mendes, who has a £41m release clause in his contract.