According to Fabrizio Romano, personal terms with Estupiñán have already been agreed, clearing the way for talks between the two clubs over a transfer fee.
Estupiñán, 28, has given his approval to a return to the Premier League after joining Milan from Brighton only last summer. Unai Emery has identified him as his first-choice option to reinforce the left-hand side of Aston Villa's defence, with the Spanish head coach eager to get the deal over the line as quickly as possible following Digne's expected exit.
Digne, meanwhile, is edging closer to Paris Saint-Germain after the French champions triggered a release clause in his Villa contract, believed to be worth around €10m, following his involvement with France at this summer's World Cup.
With personal terms already settled, attention has now turned to negotiations between Aston Villa and Milan over the transfer fee, with talks between the two clubs ongoing and progressing at pace.
Sources close to the deal say there is genuine confidence a breakthrough could arrive within days, allowing Villa to hand Emery the reinforcement identified to fill the void left by Digne's departure.
The move would represent a swift return to English football for Estupiñán, who spent several seasons at Brighton, making over 100 appearances for the Seagulls, before making the switch to Serie A last summer.
It would give Emery a defender who already knows his methods from their time together at Villarreal, offering Villa a slightly different profile to Digne alongside emerging left-back Ian Maatsen, who is now expected to become the club's long-term first-choice option in that position.
For AC Milan, meanwhile, his departure would free up space in Ruben Amorim's squad as the club continues to reshape its defensive options during a summer already defined by significant investment at the other end of the pitch.