Johnson was scratched from the lineup before Thursday's game against the Los Angeles Kings. He had an MRI earlier in the day after he was hurt in a collision with Detroit's Pavel Datsyuk in the first period of Wednesday's 5-2 loss to the Red Wings.
"A slight sprain of the right knee," said Blues president John Davidson. "It's not a long-term injury. We're relieved and he's relieved. It's a good sign for us."
Johnson, the No. 1 overall pick in the 2006 draft, has one goal and nine assists in 29 games this season. He missed the 2008-09 season after tearing the ACL and MCL in his right knee during an offseason golf outing.
Davidson said Johnson could be available to play when the Blues host San Jose on Saturday.
"We'll see about Saturday when Saturday comes around," said Davidson.