Spanish media outlet Sport, as quoted by Football.London, is reporting that Manchester United are one of a number of clubs who are monitoring Sastre ahead of a potential summer move.
The same article states that La Liga trio FC Barcelona, Real Madrid and Sevilla are all keeping tabs on the Mallorca right-back after he impressed for the Spanish club in the promotion bid.
According to the same story, Bundesliga duo Borussia Monchengladbach and Hamburg have also expressed an interest in the 21-year-old full-back in the 2018 summer transfer window.
Sastre was promoted to Mallorca first-team for the 2017-18 season and the defender helped the Spanish side secure a return to La Liga.
Manchester United have already signed Portuguese full-back Diogo Dalot in a £17m deal from FC Porto in the summer transfer window.
The Red Devils finished in second place in the Premier League table last term to secure a place in the Champions League.
Manchester United lost 1-0 to 2017 Premier League champions Chelsea FC in the FA Cup final following Eden Hazard’s winner from the penalty spot.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news