Louis van Gaal has warned Marcos Rojo he must learn to temper his aggression if he is to be a success at Manchester United.
Rojo became United's third signing of the summer earlier this week when he moved to Old Trafford from Sporting Lisbon in a £16million deal.
The 24-year-old arrives at the club on the back of a successful World Cup campaign with Argentina, who beat Van Gaal's Holland side in the semi-finals.
The defender also brings with him a poor disciplinary record of 21 yellow and four red cards in his 49 league appearances for Sporting.
Van Gaal is a big fan of Rojo, but he says the player must learn to be calmer on the pitch.
"He has to be aggressive in the right moment," the Manchester United manager said.
"When I have to say something about him that I have to criticise, that is it - he has to control himself in every moment.
"He can improve in that and maybe I can help him."
Rojo, speaking during his unveiling at the AON Training Complex in Carrington, said he was delighted to be joining a club he regards as the biggest in the world.
"When I have to say something about him that I have to criticise, that is it - he has to control himself in every moment."
Although he is now considered the enemy by Manchester City fans, Rojo revealed he has received good luck messages from compatriots Pablo Zabaleta, Sergio Aguero and Martin Demichelis, who play for United's neighbours.
"I have received messages from all of them saying welcome to Manchester," Rojo said.
"We know each other outside of football as well."
The defender's performance against Holland in the final-four clash in Sao Paulo alerted Van Gaal to his capabilities.
"I had to analyse Argentina when I was playing against him and in that video I was already enthusiastic about him," said Van Gaal, who has an open idea about where the player can slot into his XI.
"I think Marcos fits into the profile as a left central defender or a left wing-back," he said.
"He played for Argentina as a left wing-back but at Sporting Lisbon he played as a central defender.
"He had a fantastic World Cup. I am very pleased."
Rojo is unavailable for United's Premier League game against Sunderland this weekend as he is yet to receive a work permit.