"We know Aimar, of course, when he played for Valencia a few years ago, a very talented player," Ferguson told reporters when asked about Benfica's most dangerous players.
The Argentine, now 31, made his Champions League debut against United playing for Valencia in two draws with the English side in the 2000/01 season as the Spanish team made their way to a final which they lost on penalties to Bayern Munich.
Aimar, who earned 52 caps for Argentina, spent six years at Valencia, lifting two La Liga titles and a UEFA Cup. He joined Benfica in 2008 and helped them win the Portuguese title for the first time in five years in 2010.
After a lacklustre 2010/11, the playmaker has been in top form this season, flourishing in a midfield partnership with Belgian arrival Axel Witsel and combative Spaniard Javi Garcia.
Ferguson also praised Benfica's central defensive duo of Brazil's Luisao and Argentine Ezequiel Garay who will have the daunting task of containing a rampant Wayne Rooney.
The England forward has scored back-to-back hat-tricks in Premier League games, the latest coming in Saturday's 5-0 thumping of Bolton Wanderers.
Benfica coach Jorge Jesus earlier said he saw Rooney as the best English player but that his playing style was more akin to Brazilian and Argentine football - a suggestion which Ferguson rejected.
"I think he is a typical British player. The boy has great courage, wants to play all the time, has incredible stamina and added extra to the talent he has," Ferguson said.
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