The 23-year-old midfielder, who has been in inspired form as Spurs have won their opening four league matches of the season, was hurt in what seemed an innocuous collision with Lee Bowyer at the start of the second half.
He told the Croatian daily Jutarnji List: "It's like my whole world has crumbled because I will be out for at least six weeks. I am terribly sad.
"It wasn't a ferocious tackle but it hurt awfully and I knew something was wrong the moment it happened.
"The worst feeling is knowing that I will be watching both my club and country from the sidelines for a while."
"This is not the case with a fractured fibula," he said.
However, Modric will miss Croatia's World Cup qualifiers at home to Belarus on Sept. 5 and in England four days later and at least five league matches including those against Manchester United and Chelsea.
Croatia coach Slaven Bilic told local media on Sunday: "I don't need to stress what Modric means to us, I've said it many times before, but we will just have to deal with the situation and finish the job without him."
"We have enough quality players to work out an alternative and achieve our objective, which is to qualify for the World Cup.
"Obviously, it's a blow but we are not crippled and there is no room for catastrophic media predictions."
It is the second time in 18 months a key Croatia player has been hurt against Birmingham with the country's Brazilian-born Arsenal striker Eduardo da Silva suffering a horrific leg injury in a Premier League match in February 2008.
That injury kept Eduardo out for a year but Modric should return much sooner and should be fit to play in next year's World Cup finals in South Africa if Croatia qualify.
