The decision to sacrifice the Germany international, who moved to Arsenal following the Welshman's transfer from Tottenham, proved controversial among the club's fans.
And Del Piero, whose old club meet Madrid at the Bernabeu on Wednesday, was surprised that the capital club elected to move on the former Werder Bremen man.
"At first I was surprised they sold Ozil, but basically I think Madrid had to make that sacrifice so they could sign Gareth Bale," Del Piero told AS.
"I love Ozil as a player, however, so I didn't expect the club to sell such a talent."
The former Italy international also expressed his disbelief that Madrid and Spain captain Iker Casillas has been confined to a substitute's role at the Bernabeu this season.
"I find it strange to see a player like Casillas on the bench, especially when he is captain at the club," he continued.
"He is an exraordinary 'keeper and it was an honour facing him during my Juventus days."
Finally, Del Piero, now of A-League outfit Sydney, refused to pick a favourite between Real Madrid's Cristiano Ronaldo and Lionel Messi of Barcelona.
"Asking to choose between Messi and Ronaldo is like choosing between your Mum and Dad," he said.
"Messi is pure fantasy, and a special talent who can do anything, while Ronaldo is a genius who also has physical prowess. They are both incredible players."