United are hoping to write a new chapter after former Ajax boss Ten Hag was confirmed as their new manager, but the Dutchman has plenty of work to do in order to get his struggling side back to form.
"If Ten Hag managed for the rest of his life, it'll never be harder than what he's taking on now," Merson told Sky Sports.
"This is a massive job. It's the biggest club in the world and he is taking over when they're in disarray.
"He needs to bring in as many players as he can because this team is not going to get you in the top four. They need to bring certain players in - (Frenkie) De Jong at Barcelona, he's a good footballer - and then you move on from there.
"They need at least three or four players just to make a challenge for fourth place."