LISBON (AFP) - Cristiano Ronaldo returned to haunt Sporting Lisbon as his second half strike gave Manchester United a 1-0 win in their Champions League opener on Wednesday.

Ronaldo was back at Sporting for the first time since moving to United in 2003 and the Portugal winger showed his former club no mercy at the Jose Alvalade Stadium.

With Wayne Rooney understandably ring-rusty on his return to the side after five and a half weeks out with a broken foot, United were indebted to Ronaldo for yet another example of his priceless ability to conjure a winner out of nothing.

Rooney lasted 72 minutes of the Group F tie before being substituted and it was a familiar story in Portugal as the Premier League champions once again struggled to find their momentum for long periods.

But they can afford to overlook any frailties for the moment after Ronaldo took centre stage with his first goal of the season to give United a perfect start to their bid to win the Champions League for the first time since 1999.

Ferguson had no qualms about United's cautious tactics and insisted the end justified the means.

"The pitch was very difficult for both teams, that was why we had to be cautious about our game and make sure we contained Sporting," he said.

"In the first half we kept them to shots outside the box and one tremendous save from (Edwin) Van der Sar, but we had much more purpose and penetration in the second half. I thought we were the better side from then on.

"I'm delighted for Ronaldo. It was his first goal this term so hopefully it sets him off on another run."

Sporting coach Paulo Bento felt his side deserved more from the game.

"The result wasn't fair and a decisive factor was certainly Manchester United's goalkeeper," he said.

"We controlled the first half and had the best chances. They scored with their first chance and that changed the course of the game."

Success in Europe has always been Ferguson's holy grail, so it was no surprise to see the United manager pick this match to bring back Rooney.

Ferguson hoped Rooney's return would add a much-needed cutting edge to a United side who hadn't scored more than once in any game this season.

But it was Sporting who took the initiative in the early stages.

Leandro Romagnoli was first to show their intent. His powerful run caused panic in the United defence and it took a lunging challenge from Michael Carrick to deflect the midfielder's shot just over the bar.

Ronaldo was relatively subdued in the first half. His one moment of menace came in the 24th minute when he cut in from the right flank and drilled a low shot which Vladimir Stojkovic scrambled away for a corner.

United were content to play on the counter-attack, handing Sporting most of the possession. Liedson nearly made them pay for their caution after half an hour.

The Brazilian striker's curling shot was destined for the top corner until Van der Sar sprang to his left and brilliantly tipped it over.

Ferguson's decision to play Rooney as a lone striker gave the forward an often thankless task and he got little change out of Sporting's defence.

Gradually United began to find some rhythm. Ronaldo and Nani combined to give Ryan Giggs a sight of goal early in the second half, but the United captain glanced his header over.

That was a statement of intent but Sporting failed to heed the warning as United went in front in the 62nd minute.

A flowing move picked up impetus when Paul Scholes snapped a quick pass out to Wes Brown. The United right-back looked up and whipped in a perfect cross towards Ronaldo, who flung himself forward to beat Stojkovic with a diving header.

Ronaldo's celebrations were relatively restrained out of respect for his former club and he was applauded by the Sporting fans in return.

Nani, United's other Sporting old-boy, drew a fine save from Stojkovic as United went for the kill.

But to Sporting's credit they fought back gamely and it took another superb stop from Van der Sar to preserve United's lead.

Tonel met Bruno Pereirinha's cross with a header that looked in but Van der Sar parried it to Yannick Djalo, who blasted the rebound over the bar.

That was the hosts' last serious threat and United held on to claim their 100th European Cup victory and become the first English side to beat Sporting on home soil.