The 22-year-old recently secured a lucrative move to Goodison Park from Wigan and the Republic of Ireland assistant boss has challenged the midfielder to showcase his ability
The Toffees' new recruit is expected to pick up his 17th cap in midfield for Ireland's World Cup qualifier against Sweden on Friday and Tardelli has urged the 22-year-old to develop a winning mentality.
"He must show that he's a good player," Tardelli told reporters in Dublin. "Something must change in his mind. Now he's in a different team, an Everton team that wants to win."
Tardelli also confirmed that Anthony Pilkington was fit, but did not reveal if the Norwich winger was expected to start on Friday in the absence of Aiden McGeady.
"Pilkington hasn't got a problem. He's following his club training programme," Tardelli told reporters. "It's tough to decide on the players, because all the players are very fit. We have good options for Sweden."
Elsewhere, there is healthy competition for places in the centre of defence, with QPR defender Richard Dunne in line to make a return to competitive international football.
"I think so, but we have many options," Tardelli said of Dunne's potential involvement against Sweden and Austria. "He can play two matches. Richard is important for us. He's very strong physically."