The NASCAR legend appeared to grab too much kerb at the Turn 5 right-hander, and bounced nose-first into the tyre wall on corner exit, his right hand still holding the steering wheel.
The incident brought out the red flag, and Johnson initially told NBC that his hand was fine.
However, the IndyCar sophomore was unable to take part in the fans' autograph session due to a huge splint on his right forearm.
He took to social media to apologise for his absence and to explain he was heading for further examinations.
“Hey everybody, as you can see I’m a little banged up and have a splint on,” said the 46-year-old, who scored his best IndyCar result to date with sixth last time out at Texas.
“First and foremost, apologies to the fans at the autograph session that I wasn’t able to attend. I’d already got something else going on here [pointing to sore arm].
“I’m on my way to be further evaluated, just to get a good honest look and understand what my options are.
“We really don’t have any clarity at this point, and I look forward to updating you all very soon.”
Were Johnson forced to miss the race, his place could feasibly be taken by Sebastien Bourdais.
The Frenchman, who took pole for today's IMSA SportsCar Championship race in Ganassi’s Cadillac, took three straight wins at Long Beach for Newman Haas Racing's Champ Car team between 2005 and 2007.
Drivers praise track grip
Meanwhile, two-time Long Beach winner Alexander Rossi says rubber laid down by the IMSA cars was the cause of faster-than-expected laptimes during opening practice.
“There was a huge amount of grip,” said Andretti Autosport driver Rossi, who triumphed in 2018 and 2019.
“Not always does the rubber [on track] translate to performance for us.
“It was already very fast, despite it being over 130 degrees track temp.
Meyer Shank Racing's Simon Pagenaud, a winner at Long Beach with Team Penske in 2016, added: “What I noticed is on track it seems like there's new sealants from Turn 1 all the way to Turn 5. I'm assuming that is what makes the tyres work better.
“Turn 6 is actually not that grippy, but you then you go to Turn 8, really grippy.”
