Leicester made a late move for Silva on transfer deadline day as the replacement for £35million Chelsea new boy Danny Drinkwater.
The Foxes requested a two-hour extension to get the deal over the line and were confident they submitted the required paperwork in time.
However, Portuguese outlet A Bola claim his Premier League registration was completed too late.
Silva, 28, will still become a Leicester player but he cannot be registered by the club until January at the earliest.
Craig Shakespeare’s side are long-term admirers of the Portugal international, having failed to sign him last summer when they bought his former Sporting team-mate Islam Slimani.
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Leicester managed to recruit Bayer Leverkusen centre-back Aleksandar Dragovic on a season-long loan before last night’s deadline.
The Austria international joined fellow summer signings Kelechi Iheanacho, Vicente Iborra, Harry Maguire and Eldin Jakupovic at the King Power Stadium.
Silva, Sporting’s captain, has played 237 appearances for the Portuguese giants and was part of Portugal’s 2016 European Championship-winning squad.