Keane criticised the decision not to play Ronaldo from the start, arguing that his record and his recent form for United merited him being picked, regardless of the tactics being employed.
“You see a guy who’s won everything in the game, he’s come back to Man United to try and lift the club. Course he’d want to play,” Keane said.
“You still think if Ronaldo’s on the pitch, if the ball falls to anybody in world football - even at 36 - it’s Ronaldo. It’s not as though he’s come back to the club and not scored.
“Ronaldo’s not going to fix Man United’s problems, but you don’t bring him back to sit on the bench.”
The two then engaged in a furious argument over what trophies United are targeting and their current position as a club.
Carragher questioned United and PSG for their summer decisions to sign two legends of the game, and then took umbrage with Keane who argued United did not bring Ronaldo back to the club simply to win the Premier League.
“PSG have signed Messi, United have signed Ronaldo,” Carragher argued.
“Two of the greatest players of all time. Are Man United closer to the league title now than last season? No. Are PSG closer to the Champions League with Messi in the team? No.
“It’s a team game, football.”
Keane replied: “Ronaldo was never going to get United back winning titles.”
“What was the point in signing him?” Carragher asked.
“Did you like winning the FA Cup? They’re going to have to win the FA Cup, and get back in the Champions League,” Keane said.
“If you sign someone at 36, it’s to win right now,” Carragher said. “If you sign Varane, a four-times Champions League winner and World Cup winner, it’s to win right now. Are United any closer to winning the league now?
“No, he’s not come back to win the league!” Keane shouted.
“What’s he come back for then? What’s he for?” Carragher replied.
“To help them win a trophy! Mourinho didn’t win the league, but he won one or two trophies,” Keane said. “Man United are a cup team at the moment.”
Carragher then hit back at Keane’s level of ambition for United, and questioned why, if the only focus with cup competitions, they didn’t put their faith in youth instead.
“Is that where Man United are now? You’re happy with cups?” Carragher questioned.
“Absolutely! I’m not [happy], but that’s where they are as a club,” Keane said. “Look at the team, look at the league, look at the last two or three years.
“What did you bring Ronaldo back for if you’re not going to win the league for two or three years?” a bemused Carragher continued. “Start getting Greenwood on the pitch. Start getting Sancho on the pitch. What was the point?
“I’ll say it one more time - to win some trophies,” Keane said. “Some FA Cups. I know, that’s where they are.”