Ruben Amorim was involved in a heated training ground confrontation with Lisandro Martinez last month, with details emerging following his dismissal from Manchester United on Monday.
Amorim's departure was confirmed by the club following a dramatic press conference in which he criticised the INEOS leadership structure.
Now, according to The Athletic, Amorim and Martinez squared up during a training session after the Argentine defender had returned from a lengthy injury layoff.
Martinez believed he was fit enough to feature in the starting XI during the period when United faced Crystal Palace and West Ham, but felt the manager was passing him over for selection.
The 27-year-old made his frustrations known during the training ground exchange, which sources described as an intense encounter between the two.
Having been sidelined with a long-term injury, the defender had only recently resumed training and was eager to reclaim his place in the side.
Despite his protests, Martinez was limited to coming off the bench in both fixtures against Palace and the Hammers, with his first start under Amorim not arriving until the Boxing Day fixture against Newcastle.
Ruben Amorim was involved in a heated training ground confrontation with Lisandro Martinez last month, with details emerging following his dismissal from Manchester United on Monday
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However, regardless of the confrontation, the relationship between Amorim and Martinez evidently remained intact, with the defender handed the captain's armband for the manager's final two matches in charge.
This decision came about due to Bruno Fernandes being unavailable, and was seen as a demonstration of the mutual respect that existed between the pair despite their earlier disagreement.
Players frequently sought better communication from Amorim during his closing days at the club, but the former Sporting Lisbon boss was said to have responded positively when members of his squad challenged him.
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Lisandro Martinez was made captain for Ruben Amorim's final two matches as Manchester United coach
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The incident with Martinez was cited as evidence of his willingness to engage with players who voiced their concerns directly.
As of Tuesday morning, Martinez had not publicly acknowledged Amorim's departure on social media, in contrast to several of his teammates who posted tributes to the outgoing manager.
Centre-back Harry Maguire was among those to bid farewell, writing on Instagram: "Thank you for everything boss, I wish you all the best in the future."
The club released a statement confirming Amorim had left his position, noting he had been appointed in November 2024 and guided the team to the UEFA Europa League Final in Bilbao in May.
With United currently sitting sixth in the Premier League, the statement said leadership had "reluctantly" concluded a change was necessary to maximise their league finish.
Darren Fletcher will take charge for Wednesday's match against Burnley.
Darren Fletcher will take charge of Manchester United on an interim basis following the departure of Ruben Amorim
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The Scot will also speak to the media for the first time on Tuesday at 5pm.
Fletcher is, however, not expected to be in the dugout for too long.
United are reportedly planning to appoint a caretaker to oversee the rest of the season as they look to finish in the Champions League qualifying positions.
And two names have emerged as frontrunners for the role.
One is Michael Carrick, their former midfielder, who has been out of work since leaving Middlesbrough in the summer.
And ex-boss Ole Gunnar Solskjaer could be poised for another sensational return to Old Trafford, with the Norwegian willing to step up.