Fernandes said United failed to apply enough pressure and adopted a "mood of defeat" after conceding in the second minute, as the heavy loss intensified scrutiny of manager Michael Carrick.
United's collapse at Como left them staring at another damaging defeat, with only a stoppage-time goal from Joshua Zirkzee reducing the margin of loss.
The criticism follows signs of growing frustration within the squad. Earlier this month, defender Lisandro Martinez criticised the club's hierarchy after a 2-0 Champions League defeat by Atletico Madrid, saying he expected greater investment to help the team compete at the highest level.
"When Martinez came out, I said this is an absolute nightmare because it will rip the dressing room apart," Rooney told the United Stand podcast on Thursday. "That was the start. We've seen it now with Bruno and you're going to see it with more players because they don't want to take responsibility.
"You can say whether Bruno was right or wrong in what he said but you have to take responsibility on the pitch. You have to take responsibility as a group of players.
"These players, they're selfish. They don't care about their teammates, don't care about the manager, the staff or the fans, they care about themselves.
"If they're not getting what's right for them, they're like babies and just throw. their toys out of the pram and go and deflect the blame on to anyone else but themselves."
Carrick, a former United midfielder who played alongside Rooney at Old Trafford, was appointed on an interim basis following Ruben Amorim's dismissal in January before earning a contract through 2028 after overseeing a dramatic improvement in results.
Carrick is under growing pressure with United winless in seven matches across all competitions. They are 20th in the Champions League standings and 13th in the Premier League.
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