That United drew 1-1 with Everton made Solskjaer’s decision seem even more questionable.
Ronaldo, a 57th-minute substitute, stormed down the tunnel after the game. Andros Townsend had cancelled out Anthony Martial’s opener.
Ferguson appeared to question the decision to leave out Ronaldo, who has had an excellent start to life back at Old Trafford following his blockbuster summer return.
The 79-year-old was videoed in conversation in the aftermath of the draw, saying: “You should always play your best players.”
But Solskjaer has hit back at his former boss, who remains a director and club ambassador.
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“We all want to, because he’s unique, he’s different, he’s had a career and everyone wants to come and watch him.
“But the plain fact of it is he cannot play every single game that we play, that’s just physical nature, human nature.
“He knows how difficult this job is anyway, the gaffer. He didn’t always play the same eleven.
“He took some risks at times to rest Ryan Giggs, Wayne Rooney or David Beckham, the best players that we had. Sometimes it worked, sometimes it didn’t.”
Solskjaer is hoping to overcome a run of just two wins in six matches but faces a difficult run of fixtures.
United travel to Leicester on Saturday before taking on Atalanta in the Champions League next week.
The Premier League side have work to do in their European group after suffering a surprise defeat by BSC Young Boys.
They then host Liverpool and travel to Tottenham before a second game against Atalanta.
The maiden Manchester derby of the season then follows, with United hoping to gain an early advantage against rivals Manchester City.