Simon Pagenaud stands in for Justin Wilson as Briton undergoes medical tests

Wilson was taken to the Mansfield MedCentral hospital where he received a CAT scan, after initial checks at the circuit's medical centre proved inconclusive.

Team owner Dennis Reinbold suggested that the former Formula 1 driver may even be flown back to Indianapolis for further tests to ascertain the nature of the injury - which remains unspecified.

There was little visible damage to Wilson's #22 car when it came to an abrupt halt at Turn 2, having run wide on the exit of Turn 1 on the run up to the Keyhole. The car apparently ran over an adjoining crossroad, on the grass verge, and bottomed out heavily which may have caused Wilson to sustain a suspected back injury.

The Briton initially tried to climb from the car and removed his steering wheel, but slumped back into the cockpit in pain, and was extricated from the car on a backboard by the IndyCar medical team.

"I know he has had a CAT scan and there is talk of him flying back to Indianapolis for further evaluation," Reinbold told IndyCar's official website. "We don't know everything yet so we are waiting to see when we get the rest of the information.

"It looks like he will be out for today for sure so we are going to have Simon qualify the car and we are prepared if Simon needs to be our driver in the race - if Justin can't get back - then we will go in that direction also."

Pagenaud has already made one start for D&R in 2011 having stood in for Ana Beatriz at Alabama back in April and tested impressively for the team at Mid-Ohio last week.

"All those things made it easy for the choice, because there is a level of familiarity with Simon and the two parties," said Reinbold. "He has been great to work with.

"He didn't have much equipment so we got a seat from Vitor to help out, we have got Bourdais' driver suit and equipment - we borrowed things from about everybody in the paddock."