The 19-year-old Bristol Rovers winger is very highly rated, and Hull had looked to be closing on his signature.
The Tigers have been chasing Santos for months, although they could not agree terms with Rovers and were happy for the deal to go to a tribunal.
With Santos able to sign for a club from Tuesday, Hull had seemed set to conclude a deal but a late move from Derby and Celtic has complicated matters.
The Brazilian-born teenager now has options, with a decision set to be made in the coming days.
A spokesman for his agents Full Contact refused to confirm who Santos was in talks with, telling Sky Sports: “Alefe is considering his options, but we cannot confirm or deny which clubs he is talking to.”