Johnson's teammates said he initially was able to be helped off the court after running into teammate Mark Blount, but became woozy once seated on the bench, The South Florida Sun-Sentinel reported Saturday.
After Johnson underwent a CAT scan and was released from the hospital Friday, a Miami Heat spokesman announced the forward had sustained a concussion.
While Miami went on to lose the game 109-95, Heat Coach Pat Riley was focused on his fallen player after Friday's game.
"We're more concerned right now about Alex,'' said Riley, who did not say whether Johnson would miss any games due to the injury. "He got hit in the temple and he's just not responding at all. We're not sure what it is."
The Florida State standout is in his first year with the Heat, who dropped to 13-63 on the season.