Captain Damien Johnson will miss Birmingham's opening game of the Premier League season at Chelsea.
Northern Ireland are also sweating on Johnson's fitness
The midfielder, 28, suffered a tear of his hamstring in July's 2-0 friendly win against Walsall.
Johnson will also miss the home game against Sunderland, but is aiming to return for the match at St Andrews against West Ham on Saturday 18 August.
"Jonty will be out for a total of around 21 days from the time he did the injury," confirmed manager Steve Bruce.
And that means Johnson faces a race to be fit for Northern Ireland's Euro 2008 qualifier against Liechtensein on 22 August.
His absence means Bruce will have to decide on a deputy to take the armband.
Stephen Clemence stepped into the breach last season, when Johnson was sidelined for three months with a broken jaw, but the midfielder has since left for Leicester.
City won the game at Walsall on 28 July with goals from Mikael Forsell and Cameron Jerome.