The 62-year-old United manager, who penned a three-year deal at Old Trafford in May, has recruited Athletic Bilbao midfielder Ander Herrera and Southampton left-back Luke Shaw.
(Chris) Smalling, (Jonny) Evans, (Phil) Jones are all good players but they probably need a leader or captain in there to shepherd them
However, the Red Devils lost their long-term centre-half partnership of Rio Ferdinand and Nemanja Vidic after the pair completed moves to Queens Park Rangers and Inter Milan earlier this summer.
And former United midfielder Sharpe feels Van Gaal must strengthen his defence in the coming weeks to have a chance at a successful first season.
“It was very much a Dutch style of football. It can be very offensive but also safe if they need to be defensive,” Sharpe told Sky Sports News.
“But to play that formation there will be a few changes over the summer – and most probably defensively.
“If he’s going to play three centre-halves, then we may need another centre-half or two.
“(Chris) Smalling, (Jonny) Evans, (Phil) Jones are all good players but they probably need a leader or captain in there to shepherd them.
“And there will probably be another midfielder too coming in.”
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