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."
A Tottenham club spokesman said initial reports the injury would take months to heal were wide of the mark.
"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."
Croatia are second in European Group Six with 14 points from seven games, seven behind leaders England and three ahead of Ukraine who have played a game less.
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.
Spurs manager Harry Redknapp said after the game he hoped Modric had suffered only damage to his calf.
However an X-ray showed the midfielder fractured his right fibula and it could lead to him being out of action for months.
The injury comes just as Spurs have made their best start to a season for 49 years and is a major blow to the club who are second in the table with a maximum 12 points from their opening four games behind Chelsea who on goal difference.
Redknapp said he was concerned about an injury crisis at the Lane, with skipper Ledley King (groin) joining the walking wounded who include centre backs Jonathan Woodgate and Michael Dawson, goalkeeper Heurelho Gomes and midfielder Jermaine Jenas.
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