Crystal Palace and Tottenham Hotspur are just two of the sides said to have taken a shine to the 20-year-old.
Sastre has played 26 times in Group 3 of Spain's Segunda Division B - the country's third tier - which Mallorca lead by nine points with eight games remaining.
And the right-back's performances are said to have caught the eye of both Palace and Tottenham, along with La Liga giants Atletico Madrid.
Marca reports that Sastre's contract, which runs until 2020 according to Transfermarkt, contains a cancellation clause which will rise from €3 million (£2.6 million) to €5 million (£4.4 million) at the end of the season.
The Spanish newspaper goes on to add that those figures could yet be negotiated, with Mallorca aware of interest in the 20-year-old, who has been capped by Spain at under-19 level.
Palace move could make most sense...
Of the two alleged English suitors, Crystal Palace are arguably most in need of a new right-back, with the deals of Joel Ward, Martin Kelly and Timothy Fosu-Mensah - who is on loan from Manchester United - due to expire this summer.
In contrast, their Tottenham counterparts Serge Aurier, Kieran Trippier and Kyle Walker-Peters remain under contract long-term.