Leicester thought they had beaten the August 31 deadline to sign Silva, but it emerged the documentation confirming the deal between Leicester and Sporting for the £22m move arrived with the Football Association 14 seconds after their midnight deadline had passed.
The FA immediately forwarded the deal to FIFA's Transfer Matching System, which was rejected by the world governing body because the deadline had passed, but it is understood Leicester will appeal against the ruling.
"Unfortunately it's still an ongoing process," manager Craig Shakespeare said. "I'd love to be in a position to say he's available for selection but we're still just waiting for the legal processes to take care.
"So there's no further news. I don't have any hold on that, I have to wait and make sure that the people who are going through the processes do that.
"Hopefully I will know in the next 24 or 48 hours."