Salah has battled for form in the opening month of the new season, with Liverpool losing three consecutive games in the past week.
Rooney said: "With the players coming in, Isak, Ekitike, and Wirtz, the money they have spent on them, what does Salah think now?.
"Top players have an ego, and Mo Salah has been one of the best players in the league for a long time.
"I think the last week has shown that when it's gone well, you're scoring goals and you're winning games, it's great and the team will put up with that, but over the last week, I'd question his work ethic.
"I know he doesn't always get back and defend as much, but in the Chelsea game, his full-back is getting torn apart, and he is watching.
"Players like Van Dijk and Alisson, who I know wasn't playing, but the leaders in the dressing room should be telling him you need to help it. For me, that was a worry.
"He has looked a little bit lost the past week, in my opinion."