Known as "Chicharito", and wearing the nickname "Little Pea" on his shirt, the Mexican striker tapped in from close range in the fifth minute and again in the 75th as United beat the French champions 2-1 on aggregate.
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His lively combinations with Wayne Rooney, winger Nani and substitute Antonio Valencia made up for United's frailties in a makeshift defence where centre back Wes Brown headed into his own net in the 82nd minute to ensure a nervous finale.
"He (Hernandez) is a real handful," Ferguson told a news conference with the air of contentment to be expected when a good value purchase turns out to be worth a lot more.
"I think we are surprised (at how he has progressed since joining last July). When we brought him we thought it would take some time to adjust at the club, his main roles were as a substitute.
"But now he has adjusted to the physical part very well and is lasting 90 minutes very well and that's given us great options."
NEWS:Hernandez double sends United through
United paid a reported 7 million pounds for Hernandez to Mexican side Guadalajara and Ferguson has said the club's scouts saved a fortune by identifying him as a transfer target before he shone at last year's World Cup.
He has scored 16 goals this season, injected pace into the side and inspired striker Wayne Rooney to rediscover his creative touch in a deep-lying position while keeping top scorer Dimitar Berbatov on the bench.
"His movement is fantastic -- I think Wayne is thriving on that," said Ferguson, who must be delighted his strikers are coming into form after a wobble in their quest for a record 19th Premier League title with back-to-back defeats.
EXPERIENCE LACKING
The back four was threatened by Marseille who were unlucky when Andre-Pierre Gignac fired over the bar and unmarked Soulemayne Diawara headed wide with only Edwin van der Sar to beat.
Usual centre backs Rio Ferdinand and Nemanja Vidic were ruled out through injury and Chris Smalling and Wes Brown were thrown together, with John O'Shea and Rafael forced off with injuries during the game to compound the problems.
"The balance of the team moved quite a lot with two defenders coming off," said Ferguson. "There is a lack of experience there and that could have cost us."
Hernandez's verve was not just reserved for attack as having tried to charge down a goal kick he sprinted a third of the way across the pitch to intercept the start of a Marseille counter-attack early in t
