Van de Beek looks bigger after returning to preseason training, with reports claiming he worked with a personal trainer in his native Holland over the summer.
And Fernandes says the Dutchman is continuing his hard work in the United gym.
"As a professional player you always prepare for your season before you come back for training, you always do something before and I think everyone was doing their jobs at home," Fernandes told MUTV.
"I'm happy to see how everyone is, how happy they are to be back and they are training really hard."
Asked about van de Beek bulking up, Fernandes continues: "Yes he's working hard and working hard in the gym.
"Obviously he is one of the quality players we know and now he's building up."
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