The 19 year-old has quickly established himself this season as a virtual first-choice for manager Ange Postecoglou.
And Tomasson admits he's been impressed, telling Fotbollskanalen: "(His development is) incredible. It has been a tough season for Tottenham but he has had a very good year. He has taken very nice steps.
"I like that he is very humble and wants to get better all the time.
"We all know he scored an own goal against AZ, and it can happen to anyone. But the game after that he played 45 minutes and did very well. That says a lot about his mentality."
Bergvall has one goal and four assists in 36 competitive matches so far, half of which have come from the start.
The 19-year-old's contract with Tottenham extends until the summer of 2029.