Ronaldo started in his side's penalty shootout defeat to Championship side Boro on Friday.
He missed a spot-kick in normal time, but converted his penalty in the shootout - with Anthony Elanga blazing his effort over the bar to hand Chris Wilder's team a spot in the next round.
And as soon as the Swedish youngster fluffed his lines from the spot, Carragher aimed a dig at Keane over his comments about Ronaldo.
Carragher and Keane argued about the transfer of the 37-year-old last year, with the former Man United captain claiming he was signed to win the FA Cup.
Well, after The Red Devils were eliminated from the competition, the Liverpool legend fired a tweet at Keane.
"Roy Keane told me Man Utd had signed Ronaldo to win the #FACup," he wrote.
The Sky Sports pundit also shared a clip of their clash and joked about Keane's managerial interview with Sunderland.
In another post, he wrote: "'Cups, cups' #FACup I really hope Roy doesn’t go to Sunderland!"
Keane, who was in the studio for Man United's defeat to Boro, slammed his old side's showing.
"Over the last couple of home matches they have only scored one goal and one obviously again tonight," he said on ITV Sport.
"I was talking about Man United finding a way to win the game but what did they do, they found a way to lose the game. That’s not a good sign.
"You have to be critical of United. First half fantastic, but they are playing a Championship team.
"To lose to a Championship team at home in the Cup, it’s obviously not good enough."
Man United still have a chance to win silverware this season in the shape of the Champions League.
They resume their European venture this month against Atletico Madrid.
Featured Image Credit: PA/Sky Sports
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