The Dutch coach reluctantly sold the 25-year-old Belgium midfielder to Spurs for £15 million last summer and his high-intensity displays have been integral in getting his new side to third in the table.
And Jol - who has managed both clubs during different spells in England - believes that despite Bale being a hot favourite for various Player of the Year awards, Dembele is the true star of Andre Villas-Boas' side.
"We wanted to keep Dembele," he told reporters. "You can give him the ball and he won't give it away because he is too strong - even if there is no time and space.
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Spurs' win rate with Dembele in the team is an impressive 68 per cent, winning 15 games while drawing two and losing two games - but without him the win rate plummets to 14%, with one win, two draws and four defeats.
While Spurs narrowly qualified for the quarter-finals of the Europa League after going through against Inter on away goals (4-4 agg) at the San Siro on Thursday night, Jol doesn't believe tiredness will be an issue for his former club on Sunday.
"They have a big squad," he added. "If you rest five or six players, like they did at Inter, there is no problem."