Dembele was a regular player for Tottenham in his first two seasons at the club, playing 58 matches in the Premier League, including 48 starts, but the Belgium international has barely been sighted under Mauricio Pochettino this campaign.
The 27-year-old midfielder started in Tottenham's 1-0 loss at Besiktas in the UEFA Europa League on Thursday, while his last Premier League involvement was against Hull in November.
According to Ferdinand, who was assistant to Pochettino's predecessor Tim Sherwood, it is a waste of Dembele's talent, although he lays the blame squarely at the Belgian's feet.
"We used to watch him in training. Every time he got the ball he wanted someone to challenge him and he'd get past them but then just pass it sideways," Ferdinand told ITV.
With Luka Modric and Gareth Bale having traded White Hart Lane for the Santiago Bernabeu in recent years, Ferdinand revealed he had regularly told Dembele that he had the ability to follow that path.
But according to the 48-year-old former striker, who is now working with Harry Redknapp at QPR, Dembele does not do enough with his ability, which has seem him surpassed by the likes of Christian Eriksen, Erik Lamela and Etienne Capoue in midfield this season.
"We used to say to him: 'You're the next person to go to Real Madrid, you're the next guy to go to Real Madrid from this club'," Ferdinand said.
"He's got great ability, yet you just don't see enough of it.
"He leaves you frustrated because you know he's got more and you want to see it. He can get forward. He's got a great shot on him but you just never see it."
Tottenham travel to Swansea on Sunday.
