Silva and Capel have both been instrumental in Sporting's performances this season which currently sees them challenging Porto for top spot.
This season represents something of a major turnaround for the Lisbon giants, as they slumped to seventh last term - the worst finish in the club's history.
But they have galvanised themselves this time around, with Silva and Capel playing starring roles.
A host of English clubs are now reported to be keeping close tabs on both players.
Reports in Portugal claim Tottenham have checked on Silva, whilst Newcastle and Liverpool are both keen on Capel.
However, Sporting are looking to secure a high-end finish and seem unlikely to be willing to dismantle their team mid-season.
"We could not break the squad in a season with serious chances to win the title," a Sporting source told Sky Sports.
"It is possible for next summer, but any value of a player would have to be met."