The Red Devils failed to keep a clean sheet after the 2O-time English champions edged to a 2-1 victory over Arsenal at The Emirates on Saturday.
Van Gaal’s side have already conceded 15 goals in 12 Premier League games this season, managing to notch just three clean sheets.
The United manager is currently relying on youngster Tyler Blackett and Paddy McNair as Phil Jones, Jonny Evans and Marcos Rojo are sidelined.
And former Red Devils centre-half McGrath wants the United manager to sign an old head in the January transfer market.
“Van Gaal will want money to spend,” McGrath told talkSPORT. “They badly need an older, more experienced defender.
“It’s all about what you’re allowed to spend and what players will come to the club. Man United could have let in a few more goals against Arsenal, they were fortunate.”
United have been linked with Atletico Madrid defender Miranda and experienced Dutch international Ron Vlaar in recent weeks.
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