It was confirmed by police on Monday that a 35-year-old male pedestrian had been involved in a collision with a lorry on the A64 near Bishopthorpe, York, leaving him with serious injuries and needing to be airlifted to hospital.
Reports in the British media named Carlisle as the pedestrian involved and the PFA tweeted its support for the organisation's former chairman on Tuesday.
"Everyone at the PFA is keeping Clarke Carlisle in their prayers and thoughts," a post read.
"We visited Clarke at the hospital yesterday and we are offering our full help and support to him and his family at this difficult time."
Carlisle's wife Gemma, meanwhile, also took to Twitter to show her appreciation for all the positive messages the family have received.
"Don't normally use this account anymore. But have been overwhelmed with your thoughts and prayers. Thank you," she wrote.
Carlisle's career included spells with Burnley and Watford - both of whom he helped clinch promotion to the Premier League - his hometown club Preston North End, QPR, Blackpool and Leeds United before he retired at Northampton Town last year.
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