Johnson, who joined Blues from Cardiff two years ago, battled through injury to turn in another fine performance in Sunday's shock 2-1 Carling Cup victory over Arsenal and is a reported target for both Liverpool and Tottenham.
Birmingham keeper Foster, who was handed the Man of the Match award on Sunday after making string of saves, is not surprised at the interest in Johnson but is keeping his fingers crossed the highly-rated defender stays at the club for the foreseeable future.
Foster said: "Roger is always key for us. He's been a huge part of our team. You see the teams he is linked with week in week out, and it's justified really. Some of his performances have been absolutely fantastic and he's a very big player for us."
Foster added: "With the amount of money some teams have, everybody has their price. We need him playing for us, doing the job he does for us. You can see the look on his face every game he plays.
"He loves it. He loves to be in front of the firing line and getting involved, so that will hopefully keep him at the club longer."