The Liverpool legend, who speaks from experience, claims he “never, ever felt tired” during his career and in fact wanted to play in every game he could.
He suggested the bizarre tactic has the potential to do more harm than good as the Portuguese is theoretically giving his players an excuse to perform badly.
Souness added that Mourinho needs to start demanding more from his squad, who drew for the 15th time this season against Southampton at St Mary’s on Wednesday night.
“I never, ever felt tired, I think what happened if I did feel tired it would be because we lost a couple of games and you start to feel sorry for yourself, said Souness on Sky Sports.
“But that would last for, I’d like to think a nano minute, it wouldn’t last very long, I just never felt tired.
“The more success we had the more I wanted to play, I couldn’t wait for the next game to come along.
“The last thing I would want is a manager I’m working with continually telling me I’m tired.
“If you go down that road you’re constantly handing your players an excuse not to play well, I think you have got to demand it from them.”
Man United, who will finish sixth this season, take on Crystal Palace on the final day of the Premier League season this Sunday.
But Mourinho appears to already have both eyes on the Europa League final on May 24.