According to Portuguese media outlet Record, Benfica attempted to sign Estupinan at the end of the summer transfer window, but were rejected by Amorim.

In the summer transfer window that ended a few days ago, Estupinan became a focus of transfer speculation. Aston Villa, in particular, came close to signing him from AC Milan in mid-July, but Amorim halted the departure because he wanted to assess the player during the summer tour before deciding whether to allow him to leave.
Reports from Portugal indicate that the South American full-back was not only pursued by English clubs throughout the summer. Benfica also became involved. Record reported that, especially in the final weeks of August, Benfica's management made several attempts to bring Estupinan to Lisbon, but were also rejected by Amorim. Amorim was unwilling to let Estupinan leave because he believed the player was important to his tactical plans. Faced with the AC Milan manager's insistence, Benfica had to adjust their left-back recruitment target and completed a deal before the transfer deadline to sign Karouani on a loan deal with an option to buy.
Although Estupinan was initially placed on the transfer list because his first season at AC Milan had been quite disappointing, with the winning goal in the second league derby being one of his few highlights, Amorim ultimately decided to block his departure after training with him for several weeks because he believed the player could be useful to his tactical plans.
As a result, the former Brighton player has started 2 of the 3 official matches he has played for AC Milan so far, against Torino and Juventus respectively.