The two-time IndyCar champion dominated the first of Iowa's two races last weekend, and was leading the second on Sunday when a suspected failure of his right-rear shock absorber spun him into the wall.
After being cleared at the infield medical centre, Newgarden held a coherent interview with NBC but later suffered a loss of consciousness and collapsed as he walked to his RV.
To avoid road traffic, he was flown by helicopter to MercyOne Des Moines Medical Centre, where he stayed overnight under observation, before returning home to Nashville on Monday.
After observing the readings from his accelerometers during the accident, IndyCar's medical team had told Newgarden he would require re-evaluation on Thursday to determine his status for this weekend’s second of two races held this year on the Indianapolis Motor Speedway's road course.
Santino Ferrucci had been put on standby to take the wheel of the #2 Penske machine if Newgarden was not passed fit.
But Newgarden has been cleared to drive in opening practice, before undergoing another evaluation.
A statement from IndyCar on Thursday said: “The examination determined that Newgarden is cleared to participate in tomorrow’s NTT IndyCar Series practice session on the Indianapolis Motor Speedway road course.
“Newgarden will be re-evaluated after the session to determine if he’s cleared to resume all racing activities for the remainder of the Gallagher Grand Prix event.”
Newgarden, who has won four races this year, currently lies third in the points standings.
He is 34 points behind Chip Ganassi Racing's Indy 500 winner Marcus Ericsson and 26 behind team-mate Will Power with five rounds to go, with Ganassi's Scott Dixon on the same tally.
Fellow Penske driver Scott McLaughlin said earlier this week that he'd have no qualms about working with Ferrucci should Newgarden not be passed fit.
Ferrucci has driven for three different teams this year, subbing for Rahal Letterman Lanigan Racing's Jack Harvey at Texas, and Juncos-Holinger driver Callum Ilott at Detroit after racing for Dreyer & Reinbold Racing in the Indy 500.
Speaking on Wednesday, McLaughlin expressed his hope that close friend Newgarden would be fit to resume duties.
“For all of us at Penske, we really hope Josef's going to be okay,” he said.
“He's right in the thick of the championship. We hope he gets cleared to race. That's the ultimate for all of us at Penske.
“But the team has had to go out and have a Plan B – that's just part of it. We all understand that.
“I just really hope Josef's back. If it does happen that Santino subs for him, we'll work with him just like any other team-mate.”
