Johnson, who joined Blues from Cardiff two years ago, has been linked with several leading clubs and turned in another fine performance in yesterday's shock 2-1 win Carling Cup final success over Arsenal.
Foster is not surprised at the interest in Johnson but is keeping his fingers crossed he stays at St Andrew's 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."
Source: PA
Source: PA