The Belgium goalkeeper arrived from Royal Antwerp and has established himself as United's first-choice.
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Ferguson believes Lammens can prove a decisive signing in United's rebuild.
He told Press Box: "I think the appointment of the goalkeeper(Lammens) has made a difference.
"He's young at 23, he’s quick, a big lad, good feet and hands, and I think that helps.
"I think they have needed that player. United have always had inspirational players over the years. I think that has helped.
"The other two, (Bryan) Mbeumo and (Matheus) Cunha, they look as if they will contribute to the recovery of our form."