The La Liga champions sold the Spain international to Chelsea in a £70m deal this summer.
Morata, 24, impressed under Zidane last season despite getting limited game time at the Bernabeu.
The striker has hit the ground running at Stamford Bridge, netting three goals in his first four games.
And Zidane says Real may regret selling Morata this summer.
“Looking at our squad, we may be missing a No.9,” Zidane told reporters today.
“I would have liked Morata to stay but he wanted to play more first-team football elsewhere, which was his decision.
“We tried to get in another player, last year we had Morata and Mariano and now only [Borja] Mayoral is in the squad.
“Mayoral was not in the squad against Levante but that was my decision and he will be a part of our group on Wednesday - he is hungry.
“We’re happy to have Ronaldo back - physically he is very fit and mentally with all that has happened, with all the talk, he is in a good place.”
Real are in action tonight against APOEL in the Champions League.
