The striker completed a £70 million move to Chelsea, having largely played second fiddle to Madrid's forward line of Gareth Bale, Karim Benzema and Cristiano Ronaldo last season, though he missed much of the campaign through injury.
Morata was still able to score 20 goals in all competitions and, asked if Madrid are weaker for the 24-year-old's departure, Zidane told a media conference ahead of Sunday's friendly with Manchester United: "We saw what Morata did last season, he played very well and offered us a lot.
"For the moment Alvaro has gone and we are a striker short."
Mbappe has been heavily linked with Madrid, with Zidane publicly complimentary of the 18-year-old's undoubted ability.
It has been reported that Zidane had told Mbappe he would sell one of the BBC trio to ensure a spot in the team for the France international.
However, on Saturday, Zidane said: "He is a very good player and is wanted by many clubs. We'll see what happens, we have until the 31st [of August]."