He told Sky Sports News: "I'm not surprised at all. Harry is the man for a big job and he has done a big job there (Portsmouth).
"But this is an opportunity I'm sure Harry could not ignore.
"It (Tottenham) is a club he always had great admiration for. It's a big challenge in one way but it's a great opportunity for Harry so lets face the facts.
"He has done so well in football and he is looking for the new challenges and this is the one that Harry could not ignore."
Mandaric insists he has nothing but great regard for everyone involved at Pompey.
"It's a big loss for them at this time. We did well together, even with all the difficulties, but we sorted it out and after I recognised how special Harry is I brought him back. It is a great gain for Tottenham FC but a loss for my friends down in Portsmouth.
"I think they should really recognise they had a great manager, good times, he brought them there what they deserve. He left the club in good shape on the football pitch. He is a tremendous man-manager. He manages football people, he gets the best out of people.
"He's a winner and a lucky man as well."