Mexican international Hernandez added the second five minutes from time to give the English champions a commanding advantage.

"I'm delighted with the scoreline," said United manager Sir Alex Ferguson.

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"It was a very ordinary performance by us. We didn't reach any great heights in the game but we did improve in the second half and we deserved to win it."

Ajax coach Frank de Boer struck a pessimistic tone after his side's defeat.

"For us to do something against Manchester, you have to really be at 100 percent, which wasn't the case tonight," he said.

"There's a difference in class between the two clubs. I don't think we're going to win 3-0 over there."

City inflicted Porto's first defeat at their Estadio do Dragao in eight European matches to take a crucial advantage back to Eastlands for the second leg.

Silvestre Varela had given Porto -- unbeaten in the Portuguese league at home since November 2008 -- the lead mid-way through the first half.

But Alvaro Pereira's 55th-minute own goal drew the Premier League leaders level before Aguero secured victory with six minutes to play.

"I think that we played a good game," said City coach Roberto Mancini.

"We played very well. In the first half we had three or four chances to score and we were really unlucky. In the second half we played really well."

Stoke City were unable to make it a triple celebration for the Premier League when they lost 1-0 to Valencia at the Britannia Stadium.

Mehmet Topal scored the only goal in the 36th minute with a stunning 30-yard strike.

Spain's Atletico Madrid saw off Lazio 3-1 in Rome despite falling behind to a 19th-minute goal from German striker Miroslav Klose.

Adrian Lopez levelled before Radamel Falcao -- who lifted the trophy with Porto last season -- grabbed a brace for Atletico.

Former England manager Steve McClaren saw his Dutch side FC Twente edge Steaua Bucharest 1-0 in Romania thanks to a second-half strike from Ola John.

Away from the stadium, however, one Dutch fan was hurt in clashes in a Bucharest bar between rival fans.

There was also trouble in Italy when a steward was hospitalised after fans pushed over a fence before the goalless draw between Udinese and Greek side PAOK.