The former Gunners defender labelled the Red Devils boss as arrogant and believes the Dutch manager's judgement is now under scrutiny at Old Trafford, following his failure to get the best out of Radamel Falcao and Angel di Maria.
Van Gaal took Manchester United back in to the top four in his first season and Keown admitted he is a big admirer of one of United's new signings.
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When asked about which manager needs to live up to the hype, the former Arsenal defender wrote in his column in the Mail: "Louis Van Gaal. He’s quite an arrogant man. He’s spent a lot of money, failed to get the best out of Angel di Maria and Falcao and his judgment is now questionable. Will this year’s signings fare any better?"
He added: "Manchester United's XI looks like it will change more than that of any of the other serious contenders. Bastian Schweinsteiger can finally replace Paul Scholes and I’m a fan of Morgan Schneiderlin, too. Louis van Gaal said the squad was broken so maybe he has fixed it now."
United have made five new additions this summer as Van Gaal looks to improve on last seasons fourth place finish. The revolving door at Old Trafford shows no sign of stopping this summer with David de Gea seemingly on his way out after it was announced he will be on the bench against Tottenham Hotspur.
Following the sale of Angel di Maria to PSG it is also expected Pedro will join the club before the transfer window slams shut.
It is too early to say whether these new additions will take Manchester United back to the top of the pile, but having addressed their glaring weakness in central midfield United appear to have a more well rounded squad for the upcoming season.