The London club's former midfielder believes that the 31-year-old and a number of other players may have moved to Loftus Road because of the wages that are being paid by owner Tony Fernandes.
Cook said: "Every time I saw Park at United, he was a workhorse.
"For me, he doesn't seem to be working as hard as he did there.
"People I know think a few of them are there for a pay-day. At halfway through the season, you will be able to tell whether this is true or not."
QPR have yet to pick up a victory in the Premier League this season.