The Belgian midfielder has said it has helped them score a couple of crucial, late goals of late.
Fellaini appears to have been Louis van Gaal's plan B in recent weeks when United have been in need of a goal, using the height, presence and strength of the Belgium midfielder to cause problems for their opponents.
Lately United have discovered something of an old knack they had when managed by Sir Alex Ferguson with late goals, with Daley Blind rescuing a point for them against West Ham United, and Ashley Young's late strike against Newcastle United to keep them two points clear in the Champions League places.
Fellaini told MUTV: “We always keep going and scored in the last minute. The spirit was good, we continued [probing] and that is important.
“We have showed it already a lot of times now and we have to continue like that. We have a big game on Monday, so it is good for the confidence.”
United face their stiffest test in the FA Cup on Monday night when they take on Arsenal having come through against lower league opposition in the form of Yeovil, Cambridge and Preston, who all give the Premier League side a run for their money.
If there were to produce another late show here, or any time in the near future, then there would be talk of that quality that made them so feared among their opponents, returning.
But they will want an improved performance against Arsenal, and can take heart from their win at the Emirates Stadium earlier on in the season. Though not particularly convincing against the Gunners in November, they got the job done - and a repeat is something United will settle for.