The Red Devils have endured a difficult start to the new season and threw away a two-goal lead as they were beaten 5-3 by Leicester City on Sunday.
"It doesn't take a great tactical mind to go out and sign Radamel Falcao and Angel di Maria. Any United fan could have done that. It's not rocket science. But fixing the United defence is another matter. Now we really will find out if Louis van Gaal is a genius," he wrote in his column for the Daily Mail.
"Everyone could see that United were light years away from where they should be. They were so obviously short of top defenders and have given themselves a major problem by not signing a recognised centre-back.
"They had no Champions League in the week and plenty of time to work on their defensive system. There were no excuses and to concede five goals against a newly promoted club is appalling."
United are currently 12th in the Premier League table.