The Egyptian great was again ineffective on Saturday as Nottingham Forest won 3-0 at Anfield.
"If I was Slot, I'd try and make a big decision just so it has an impact on the rest of the team," said Rooney in his latest podcast episode.
"Salah is not helping them defensively.
"If you're one of the players who they've signed and you're on the bench and you've seen him not running - and again he's a club legend and everything he's done for the club - but if you're on the bench, then what message does that send to you?"
Liverpool sit 11th on the Premier League table, with 18 points from 12 games.
"They're going through a hard time, aren't they?" added Rooney.
"You've probably got to look at the effect of what sadly happened to Jota. What effect does that have on the players because that's his team-mates? That surely has to have an effect.
"But then there's no excuse for not fighting and not tackling."