The Reds were beaten 2-1 at home by United on Sunday night.
Rooney insists Slot was wrong to start Alexander Isak ahead of Hugo Ekitike, with the former being hooked in the second-half.
"I wouldn't play Isak, he hasn't looked ready since coming from Newcastle," Rooney said on The Wayne Rooney Show.
"He hasn't trained, hasn't had a pre-season. It's so important. While Newcastle were training he was probably sat at home on the phone to his agent for six hours a day trying to get a move.
"It's so difficult when you don't have a pre-season. He might've done stuff by himself but he's paying the price of it.
"On performances, he doesn't deserve to be playing ahead of Ekitike."
On Slot, Rooney further expanded: "They've got a bit complacent.
"For Arne Slot, you forget because he won the Premier League, but he's still quite young - 46 years of age and this was the first time he's lost four in a row in his career.
"He was arguing with the officials on the touchline, I haven't seen that before from him.
"We're starting to see things unfold which shows that there's pressure there."