Dier had six months at Goodison Park.
"I had become a bit spoiled or complacent at Sporting and I needed a kick up the backside, which is what Everton gave me. It was quite tough going to an unfamiliar place where I didn't know anyone at first," Dier told the Daily Mail.
"Coaches like Alan Stubbs were quite hard on me but that helped a lot. It was the best thing that could have happened. It really hardened me up physically and mentally. The football was much faster, physical and more direct than here."