Johnson, the Blues' top draft pick pick in 2006, was trying to stop his golf cart on Tuesday when he jammed his right foot between the accelerator and brake and felt pain in his right knee, according to the St. Louis Post-Dispatch.
While Johnson was able to finish the round, he noticed swelling in the knee on Tuesday night, according to the report. The swelling got worse on Wednesday, but an initial MRI exam was inconclusive and a follow-up MRI exam is expected Sunday or Monday, the Blues said.
"It's swollen and all I can do is ice it," Johnson said, according to the report. "I can't do anything about it ... It's just a freak incident, and it's tough to deal with, but hopefully it's nothing."
Johnson's status will be updated before Tuesday's preseason game against the Dallas Stars.
The Associated Press contributed to this report.