With Manchester United also keen, Milan have made a first move for the Ecuador international.
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Sky Italia says Milan's bid has been rejected by Brighton.
While the offer hasn't been revealed, it's understood there is room for Milan to make an improved proposal.
The Ecuadorian's contract with Brighton runs until the summer of 2027.
Brighton are said to be seeking £40m to sell Estupinan, with a first United bid soon expected.