Mauricio Pochettino appears powerless to prevent Mason Mount from leaving Chelsea (Picture: Getty)

Paul Merson fears Chelsea have already made the first ‘mind-blowing’ mistake of Mauricio Pochettino’s tenure by allowing Mason Mount to join Manchester United.

The England international has just over 12 months remaining on his current contract and with the Blues’ hierarchy unwilling to offer him a deal that would draw him level with the club’s top earners, Mount has taken the decision to leave his boyhood club.

Liverpool and Arsenal have expressed an interest in the attacking midfielder but United look set to win the first significant transfer battle of the summer.

The Red Devils are said to have agreed personal terms with the 24-year-old and although there is a discrepancy around the transfer fee, negotiations between the two clubs are ongoing and a breakthrough is reported to be imminent.

Pochettino was understood to be keen on retaining Mount’s services and Merson is stunned that Chelsea have opted against ceding to the player’s demands and risk upsetting their new manager.

Mason Mount appears to have played his last game for Chelsea (Picture: Getty)

He told Sky Sports: ‘I cannot believe Chelsea are letting Mason Mount go.

‘We saw it last season with Antonio Rudiger, who was outstanding and one of the best defenders at the club. He asked for more money and they didn’t give him it and then they buy Kalidou Koulibaly for £33m on a four-year contract which works out as more money than what they’d give Rudiger.

‘Now they’re doing it with Mount. He’s one of their own, he hasn’t had a very good season but they’re judging it just on one season. The year before he was Chelsea’s best player. He was one of the first names on the teamsheet for England. Now they’re letting him go. It’s mind-blowing.

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‘They’ll bring someone in who will probably take another year to get used to the league because they will have played abroad.

‘You’re also letting a very good footballer go to Manchester United or Liverpool, who Chelsea are fighting with to get into the top four next season. You couldn’t write it.

‘How can you let him go to a club that could majorly haunt you? United won’t believe their luck if they get him.’

MORE : Frank Leboeuf tells Chelsea to make shock move for Fulham star and sell £97m flop Romelu Lukaku

MORE : Manchester United open talks with Chelsea to seal move for Mason Mount

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The former Tottenham boss has walked into a very difficult situation.