Manchester United boss Sir Alex Ferguson has conceded that he's struggled at times to keep Cristiano Ronaldo focused.

In an interview with CNN to be broadcast on Friday morning, Ferguson admits he has sometimes struggled to prevent Ronaldo's head from being turned.

"I think that he's had temptations and I think there have been periods when you've seen the effect of it (on the pitch)," the Scot said.

"He's lucky in the sense that people here can tell him the truth and be straight with him and he appreciates that, and he has responded to that.

"If it stays with him and changes him as a person then you have a problem," he said. "Fortunately Ronaldo has good human qualities. He's a good guy.

"He knows when he's gone too far and he draws back in after that."