De Ligt completed his switch to the Serie A champions on Thursday and has penned a five-year contract, which will keep him at the Allianz Stadium until 2023.
The 19-year-old captained Ajax to winning the Dutch domestic double - the Eredivisie and Cup - last season and also guided them to the semi-final of the 2018-19 Uefa Champions League campaign, where they bowed to Tottenham Hotspur on away goals after a 3-3 draw.
The duo struck a fine defensive partnership last term, which helped the Sons of Gods keep 26 clean sheets across all competitions.
Onana hailed the Dutch international as his brother and described the moments shared together in Erik ten Hag's team as "an honour".
"It has been an honour to share the pitch with you, my brother. Take care, Captain," Onana tweeted.
The Indomitable Lions bowed out in the Round of 16 after a 3-2 loss to Nigeria.