Warnock and Barkley linked up at Elland Road in 2013, when Barkley was still trying to prove he was ready to make the grade at Everton. And back then, the 66-year-old boss felt that Barkley was very weak defensively.
It is a fault he feels Barkley hasn't entirely eradicated, and he has warned England boss Roy Hodgson of what to expect if he does select him at Euro 2016.
"I had him at Leeds on loan and the problems I had then, Roy is going to have," Warnock said.
“He has got super talent, but he can’t defend, he can’t take responsibility in his own half.
“If you play Ross, you have got to let him have his way, you have got to let him have that free role behind a striker, the third midfield man if you have to with two good anchor-men. But you can’t give him any responsibility and, if Roy is in doubt, he will go for the defensive type of player. I fear the worst for Ross."
Despite Warnock's less than glowing appraisal, Barkley has been in wonderful form for club and country so far this season.
The 21-year-old has scored five times in his first 14 appearances for Everton and England, and is starting to develop into a very refined player.
Barkley looks likely to go to Euro 2016 with England, providing he can maintain his current standards and steer clear of injury, and he will be hoping to prove his old manager wrong in France next summer.
During his time with Leeds, Barkley made just four appearances and rarely produced his best for the Yorkshire club.