Manchester United boss David Moyes has told MUTV that his first impression of Wayne Rooney when he started at the club was that he had "gone a bit soft".
Rooney's future was the subject of intense speculation over the summer, but, in spite of interest from Chelsea, the striker remained at Old Trafford.
After question marks over Rooney's commitment to the Premier League champions, the England international has recently impressed for the club, scoring three times in his last two appearances.
"I thought he had to get back to being a bit more aggressive. We are challenging him to be that at the moment because when he is, he is a handful to play against."
Moyes continued: "Wayne's attitude is that he is a big-game player. People come to see people like him make the difference.
"The top players do influence games. That is why we are hoping that having players like Wayne Rooney at our club gives us a chance of making a difference when games are tight."
Moyes said: "I didn't think there was ever a line to draw under between me and Wayne but now he realises the situation.
"This is a big year for him with the World Cup and he would say, by his standards, last season wasn't my best season.
"He has set out his own stall to say he is going to improve on that."