'He was cocky and got ripped apart. He learnt a lot of lessons'

Renowned for a relaxed demeanour that once threatened his international career, Trent Sainsbury had the heat turned up on him at Central Coast Mariners.

Graham Arnold made sure of that. On and off the pitch.

As the man who gave an 18-year-old Sainsbury his first professional contract elaborates, he had reason to go hard during their three successful seasons together in Gosford.

"He's married to my daughter now but I didn't know anything about the relationship for six months," Arnold said with a smile in an interview with FFA TV.

WATCH THE FULL INTERVIEW ON TRENT SAINSBURY IN THE VIDEO AT THE TOP OF THE PAGE

"I think everyone else on the Central Coast knew except for me.

"When I found out about it, I made his life tougher."

It wasn't the only way he applied pressure to the talented young defender.

READ: FIFA World Cup Ultimate Guide - Matches

In just the second game of his Hyundai A-League career, Sainsbury was made to start - and suffer - against a rampaging Brisbane Roar and their German wing wizard.

"I remember one game in the first year, I put him at right-back, because he was cocky, to play against Thomas Broich," Arnold said.

"Broich ripped him apart and I made him stay out there for 90 minutes, and he learnt a lot of lessons.

"He's got a very laid-back attitude. Unless you know him it comes across as a 'who cares' attitude. He went to one camp with one Holger Osieck, the [2013 EAFF East Asian Cup], and he came back and the report I got back from Holger was he'll never make it, because he doesn't care.

"I spoke to Holger about it. I said I've worked with him for three years, I know his attitude and he does come across like that. He does care, you've just got to be harder on him. 

"I was always harder on him because of expectation. When you have that gift he has and the speed, technically very good, it's about getting it out of him.

"He needed to be taught in a certain way, and that was about being on the pitch with the best players."

It's for that reason Arnold views his son-in-law's next club choice as a "big move" in the context of his career.

His loan spell with Swiss side Grasshopper Zurich is due to finish during the FIFA World Cup, leaving him with a decision to make over whether to return to Chinese parent club Jiangsu Suning.

Impressing in Russia could put a player headed for his third major international tournament in the frame for European football's top-tier, an environment he experienced in a short stint at famous Serie A outfit Inter Milan last year.

READ: FIFA World Cup Ultimate Guide - Broadcast

Arnold believes the "skinny kid with a rat's tail" he, alongside Pim Verbeek, first saw playing for the AIS on the Gold Coast is yet to unlock his full potential.

"If he plays at his Socceroos level every game for a club team then he'll be at a very, very good club," he said.

"He needs to be challenged every week, every game, and I always believe the better the opposition, the better Trent plays.

"When the challenges are there – I think you'll see it at this World Cup – when the challenges are there I think you'll see him go to an even higher level than what he's at."