Manchester United boss Ole Gunnar Solskjaer has been urged to spend £180million on transfer targets Jadon Sancho and Declan Rice.
Hargreaves, speaking after United's 1-0 win over Astana in the Europa league on Thursday, said Solskjaer must also sign a centre forward.
And he urged the club to match rivals Liverpool and Manchester City in the transfer market if they want to return to the top of English football.
Borussia Dortmund winger Sancho is believed to be United's top target for next summer, with Solskjaer seeking young British players with a hunger to succeed in the game - but he could cost around £100m.
And West Ham midfielder Rice, who has long been linked with the Old Trafford outfit, could cost another £60m on top.
"Sancho would be a perfect player for this team," Hargreaves said on BT Sport. "And they need a No.9 - whoever that might be.
"At some time they are going to have to invest because we have seen City and Liverpool, they've bought players who have made a real difference.
"Eventually you have to buy. Declan Rice fits in here, whether he costs £50m or £60m. He fits the mould of young and British.
"Jadon Sancho is nailed on for this team. Even if he costs £120m, he is there for a decade so you almost can't overspend on him.
"When you buy, buy players who will play. They cannot afford those misses anymore."