Sir Alex Ferguson believes Cristiano Ronaldo should have started Manchester United's game against Everton, saying his absence provided the Toffees with a boost as they claimed a 1-1 draw at Old Trafford.
Ronaldo began Saturday's game on the bench, with manager Ole Gunnar Solskjaer claiming his team needing refreshing after their "physically and emotionally" tiring win over Villarreal in the Champions League three days earlier.
Ronaldo was brought on during the second half against Everton - when United were winning 1-0 - but was unable to help them hold onto the victory.
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United have now won just two of their last six matches and Ronaldo looked frustrated at full-time on Saturday, walking straight down the tunnel while muttering to himself.
Ferguson - who won 13 Premier League titles during his reign at United - was in attendance at Old Trafford on Saturday and was filmed after the game talking to UFC legend Khabib Nurmagomedov.
In a video posted on social media by the Russian, Ferguson is heard saying Everton were given a boost "when they saw Ronaldo wasn't playing."
After Nurmagomedov - who retired with a 29-0 record last year - points out Ronaldo came on in the second half, the Scot replied: "Yeah I know... but you should always start with your best players."
Manchester United are not commenting on the story.
Ronaldo has scored five goals in six games since returning to United from Juventus this summer.
He was first signed for the club by Ferguson when he was an 18-year-old at Sporting Lisbon in 2003, before being sold to Real Madrid for a then-world record fee six years later.
Ronaldo has said that Ferguson played a role in convincing him to make his dramatic return to Old Trafford this summer.
Gary Neville says Ronaldo's actions at Old Trafford serve only to increase pressure on Manchester United boss Solskjaer.
"I watched the game and I watched him walk off. I can't say I liked it," Neville said on his latest podcast.
"Is Cristiano upset when he doesn't play? Yes. Is Cristiano upset when he doesn't score? Yes. Is Cristiano annoyed as hell when the team don't win? Of course. We know those things. He doesn't have to prove that.
"He walks off the pitch, muttering to himself, which throws questions up in the air. What is he saying? Who is he annoyed with? It can only come back to the manager.
"Cristiano is smart enough to know actions like that will bring real pressure on the manager - more than he is already under.
"It is what it is, but I think that is something that has to be managed in the next couple of months.
"Cristiano is not always going to score and he maybe can't play every game. But if he's going to walk off like that, it's going to put immense pressure on Ole Gunnar Solskjaer."