That's the view of Watford cult hero Troy Deeney, who believes the 25-year-old isn't coping with the great expectations at the Bernabeu.
The talkSPORT pundit has also suggested that Los Blancos may live to regret snapping up the Frenchman on a free transfer after the forward ran his contract down at Paris Saint-Germain.
In 16 appearances for the LaLiga giants, Mbappe has banged in eight goals and chipped in with two assists from 16 matches in all competitions.
While he is the club's second top scorer this season behind Vinicius Jr [12 goals], Mbappe has come under fire from fans and pundits over his inconsistency.
Three of his goals for Madrid have come from the penalty spot, with many blaming Mbappe for the club's mixed start to the season.
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Despite Real beating Atalanta the UEFA Super Cup - which saw Mbappe open his account on his debut - they sit six points behind LaLiga leaders Barcelona.
And Carlo Ancelotti's men languish in 18th spot out of 36 teams in the revamped Champions League table having lost two out of four matches.
And according to Deeney, Mbappe must deal with the pressure of playing for Real if he wishes to succeed.
Commenting on Mbappe's current form when speaking to talkSPORT.com, he said: "It's probably not been anywhere near where we all expected.
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"I thought personally when they got him that he was the missing piece, a bit like when [Manchester] City got [Erling] Haaland.
"You expect him to score a lot of goals and for them [Real] to be more or less unstoppable.
"What I think has happened is, and I don't know this to be factual, but just looking at it, my opinion is his ego has gone before his actual work ethic.
"Being at PSG he had the freedom to do whatever he wanted, and also the expectation wasn't as great, they were always going to win the league.
"So going to Real Madrid, there's Barcelona, there's Atletico [Madrid], there's the expectation you win the league and the Champions League.
"And I think at the moment that's just weighing a little heavy on him."
Deeney then suggested that despite being signed to lead the line for Los Blancos, playing in a central role up top is not getting the best out of him.
The ex-Birmingham striker's view comes amid claims that Mbappe prefers to be deployed on the left wing - which is currently occupied by Vini Jr.
He remarked: "I don't think he's a natural striker, I know that sounds mental.
"He can't [play on the left wing] because Vini Jr is there, so in that situation, it's a massive mistake from Real Madrid.
"They've gone for a name that sells shirts and the brand as opposed to what makes them win.
"So maybe he's going to have to have a little conversation about playing on the right."
Mbappe rejected the chance to extend his stay at Parc des Princes such was his desire to link up with Madrid, with it being his dream transfer.
And Deeney has stated if that is the case, then he must work harder to win over the fans and get back to his best.
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He stated: "If it's his dream, he needs to start playing down the middle and actually learning to stay onside.
"When I watched him against Barcelona, Jesus Christ, he was offside about nine times.
"You're rapid, it really irks me especially when players are quick, probably because I'm f****** slow as s***, these guys that are quick don't bend their run, it's like they're overeager.
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"In that regard, if it's his dream, he needs to drop his ego and work a bit harder."
Mbappe will soon be displaying his talents in England following the international break when Real face Liverpool at Anfield in the European Cup - where all eyes will be on him.