United have suffered a dip in form recently, being well-beaten at Tottenham before a narrow win over Huddersfield last weekend.
They make the trip to Newcastle on Sunday looking to hold off their top four rivals.
But Merson believes Jose Mourinho and co are in for a nervy weekend against Rafa Benitez’s Magpies.
He told Sky Sports: “United are just efficient, aren’t they?
“They played a weakened Huddersfield at home and fell over the line – you could not come off that pitch and say they could have won by nine, but they were efficient.