Everton are braced for an improved offer from Chelsea for Stones after turning down an initial £20 million (Dh113.8m) bid and ex-Barnsley defender Hassell said a move to Stamford Bridge would not daunt the 21-year-old England international.
“I could see him going to play for Mourinho and getting even better,” Hassell told Press Association Sport.
“Natural defending is what Mourinho is a master at and John’s got everything else – if he was a foreign player everybody would be raving about him.
“He’s that good on the ball, he’s quick and so comfortable in possession.
“He’s a good kid and a level-headed lad. He’s from Barnsley and his parents are good people so he’ll deal with the pressure that will come his way, unlike a lot of other lads.”
Hassell, who had his Barnsley testimonial last weekend against another former club Mansfield, helped nurture Stones when he made the first-team breakthrough at Oakwell at the start of the 2012/13 season.
Everton had seen enough potential just halfway through that season and paid £3m for Stones the following January.
“I played with John and was his mentor alongside Stephen Foster,” Hassell added.
“We’re still good friends and I’ve just texted to wish him all the best if he does go to Chelsea.
"When he was 15 I said to people at Barnsley he would go on to play in the Premier League and for England and they laughed.
“Not everyone saw a world-class defender there – he might not be yet, he’s still got a lot to prove.
“But for a modern-day footballer he’s got everything – 6ft 4in and very athletic, very quick, loads of stamina and so good on the ball.
“It’s his athleticism that sets him apart. In the Premier League players coming through at 18 are no better than those in the Championship ability-wise.
“But they’re complete athletes, quick and tall and John is all of that.”
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