Holtby had been expected to make the move to White Hart Lane in the summer but Spurs have pushed through the deal early after reaching an agreement with Schalke.
He has cost a reported £1.5million and Villas-Boas thinks that represents a wonderful piece of business, while he says the 22-year-old's versatility will stand him in good stead.
"Is he the bargain of the century? I think so because it's difficult to find a young player who is running his contract down and we faced tough competition," Villas-Boas said in the Daily Star.
"He's recognised as an exceptional young talent and we have high expectations of the player.
"Because of his creativity you can compare him to Rafa - to a certain extent anyway because Rafa was a greatly experienced player when he joined and Lewis has all his career ahead of him.
"He will fill the No.10 position, although he can play on the right or the left and he has also played in a holding role."
Holtby could make his Tottenham debut in Wednesday's Premier League game against Norwich City.
Villas-Boas said: "He's been selected. Is he ready to go straight in? It depends, but his English background can help him and he knows the culture and the language very well."
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