Lisandro Martinez’s furious rant at the Manchester United hierarchy was reportedly sparked by the decision to play Ayden Heaven at left-back.The Argentine defender called out the Red Devils’ director of football Jason Wilcox in an explosive interview following the 2-0 defeat to Atletico Madrid on Tuesday in the Champions League.Martinez did not hold back on his assessment of United as he called out the board for their transfer businessGettyMartinez slammed the Man United board for a lack of investment during the summer transfer window.The 28-year-old doubled down in a statement on Wednesday, insisting: “My words reflect how I feel,” before issuing an apology on Thursday evening.And the root cause of his public outburst has now reportedly been identified, with Michael Carrick‘s backline selection causing an issue.Heaven decision angers MartinezWith Luke Shaw United’s only senior left-back option, his absence from the Atletico clash forced centre-back Heaven to deputise.According to the Sun, the decision angered Martinez, who felt asking Heaven to play out of position proved the board has failed to provide a squad capable of competing at the highest level.Left-back was identified as a priority position in the summer by Carrick, but he was unable to land top target in Newcastle’s Lewis Hall. With this deemed too expensive a move, the Red Devils then saw their deadline day enquiry for Manchester City’s Rayan Ait Nouri dismissed.Instead of strengthening at left-back, United spent just £153m on midfield trio Carlos Baleba, Youri Tielemans and Andrey Santos.But with United now set for weeks without a specialist left-back after Shaw limped off in their 1-1 draw with Tottenham on Saturday, Martinez was left angered by a lack of squad depth.A source told the Sun: “Martinez was furious because he thinks having to play Ayden at left back away in Madrid shows how ill-equipped the squad is. It doesn’t have enough depth.Ayden Heaven has made just 30 senior appearances for United and played in an unfamiliar role against AtleticoGettyMartinez did not hold back on his assessment of United as he called out the board for their transfer businessGetty“And the fault of that lies squarely with the board.”The source added that Martinez was not criticising Heaven directly, but rather the 20-year-old not playing in his best position.While Martinez initially appeared to stand by his explosive interview comments, he has since revealed that he and senior personnel at United have patched things up.In a statement on Thursday, the 2022 World Cup winner said: “Today I met with the club’s leadership and the manager. It was a very positive conversation, where we were able to exchange views and speak openly about what happened after Tuesday’s game.“Everyone knows that I am a passionate person and that I live football with great intensity. The frustration of the defeat led me to express how I felt at a time and place that perhaps were not the right ones, and for that I want to apologise.”talkSPORT understands that Martinez has been fined by the club.Martinez has been fined by United for his outburst, but all parties consider matters to be closedGettyHas Martinez reaction been justified?Martinez’s explosive comments have had a mixed response, with some sympathetic to his frustrations amid a lack of summer transfer activity at United.Meanwhile, the club’s all-time leading goalscorer Wayne Rooney slammed the Argentine’s rant as ‘disrespectful to his teammates’.But former Manchester United assistant coach, Rene Meulensteen has claimed there has been an overreaction to Martinez’s outburst.Speaking on talkSPORT Breakfast, Meulensteen said: “I think some things have been taken out of context a little bit because I don’t think that Martinez said anything bad about the current transfers or the players, he just said that they weren’t backed enough. “And sometimes after football [matches], you know, the emotions run high, they haven’t won in five games.“I think it was just an outburst of emotions and negativity where he felt that possibly again Michael Carrick was coming under scrutiny, maybe he wanted to protect him, but it is not to say that I agree with what he did, absolutely not.Rene Meulensteen served as assistant manager at Man United between 2008 and 2013Getty Images - Getty“But sometimes you need to look at what he said, and I think if you take that into context, I think there’s plenty of people that would agree with him. “So, yeah, what is going to happen? Obviously, he apologised, then you have to sit down with him.“I don’t think he necessarily put Michael Carrick in a bad position, but Michael has to obviously deal with it and make sure that maybe in some ways [he] uses it to his advantage, because the only thing that needs to happen now with Man United, everybody has to step up.“Everybody has to step up and the best way to do it is on the against Leeds.”