David Wagner admits Huddersfield will be prepared to foul Alexis Sanchez again in the next round of the FA Cup.
Huddersfield set-up a televised fifth round home tie with Manchester United after this extra-time win at Championship Brum.
Steve Mounie headed his sixth of the season in the 94th minute as the Premier League visitors eventually progressed.
Sub Rajiv van La Parra struck in the 97th minute with a follow up after Tom Ince’s shot was blocked.
Ince deservedly got on the score-sheet himself in the 106th minute with a superb shot from outside the area.
Now Town face United again in the FA Cup later this month after a 2-0 loss at Old Trafford at the weekend when new-boy Sanchez was fouled a record seven times - more than any other player this term.
Wagner, whose side beat United at home in October, said: “Every win helps to build up momentum and lift confidence.
“We are very happy to be in the next round to play Manchester United at home.
“The fifth round is never a distraction , it is something not only the players but the supporters and whole town can enjoy.
“If he (Sanchez) will be on the grass then we try to defend him and if this means sometimes he is too quick , we have to foul him.
“It is not something that we like to do but this happens and it is because he is a top quality player.”
Che Adams made the breakthrough in the 52nd minute for the hosts with a low right-foot shot from the top of the box following a Lukas Jutkiewicz knock-down.
It was the striker’s sixth of the season for Steve Cotterill’s improving Championship strugglers.
But Town were soon level in the 60th minute after Ince hit a fierce shot which David Stockdale initially saved but it spun over the line as Marc Roberts tried in vain to clear it.
Stockdale repeatedly denied Huddersfield in normal time but their Premier League class and fitness prevailed.
Blues boss Cotterill said: “The lads were dead on their feet by the end. We needed another goal in 90 minutes but I couldn’t be prouder of that dressing room because they have given us everything.”
Birmingham visit arch-rivals Aston Villa on Sunday next in the Championship.
Cotterill added: “We will be looking forward to that to go and do our best there.”