Johnson was a key player in Birmingham’s Carling Cup final win over Arsenal at Wembley on Sunday.
“Roger is always key for us,” said Foster. “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.”
