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The winger scored moments into the second half to round off a good team move and ensure Jose Mourinho's side came away with the victory.
"Having the faith of the boss is important, one of the most important things," Zalewski said.
"We put in a good performance tonight against a good side. It was important to win, that's what the coach asked us to do in the changing room. I think we are pleased with how this latest test went."
He added: "We are working a lot on our speed, on our pressing high up the pitch. This year we are really working on those aspects. We will try to have a great season."
Carles Perez, who scored in each of the club's opening two training games of pre-season at Trigoria, joined Zalewski in making a second half appearance.
"I feel good, I feel in shape," the Spaniard said.
"I'm ready for however the coach wants to use me. [Mourinho] is trying to build a winning mentality in us. We still have time to keep improving, but things are going well."
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