They lost to relegation-threatened Sunderland before Thursday's 2-1 Europa League defeat by Danish club Midtjylland and Smalling wants a positive reaction from his team mates.
"The aggression that you start the game with is something you need to match with the other team, something I don't think we have done recently," he told the club's website.
Van Gaal, heavily tipped by British media to be replaced by former Chelsea manager Jose Mourinho, said his team needed a reaction in the fifth round cup match against Shrewsbury.
"We have to make them fear again," he said. "You cannot continue with losing games and the question is how we lift ourselves."
(Reporting by Simon Jennings in Bengaluru. Editing by Patrick Johnston)
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