The 53-year-old is confident his side can upset the odds against the Red Devils, but has praised the physicality of Jose Mourinho's side.
United won their opener in the Champions League against Young Boys, while the La Liga side lost to Juventus at home.
Speaking ahead of the Old Trafford clash, Marcelino said: "They are one of the three physically strongest teams in Europe.
"They have a very high potential in attack, with players who cost €70million (£62m), they are very strong in the air, prepared and conditioned to fight for the maximum.”
Before adding: "We will try to counteract their play with a collective effort. They are powerful in duels, but they struggle to defend collectively.
"We will win if we play well. Is it a good time to play them? Who knows. We know we are facing a difficult opponent, maybe they are not in their best moment, but it could be similar for us."
Just like opponents United, Valencia have also endured a tough start to the campaign.
They picked up their first win of the season against Real Sociedad on Saturday to climb up to 14th in the table.
One person looking to prove to be a nuisance at the Theatre of Dreams will be former Bolton loanee Rodrigo.
The Real Madrid target said of United's current form: "They are a team unused to being in situations like this.
"They have great players who are not used to this, but that is a career as a footballer. I'm sure they will want to turn things around as soon as they can."
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