If there was any doubt about the scale of the task facing the Dutchman, it was demonstrated by last weekend's 2-1 opening day defeat at home to Swansea.
Sunderland, who started the season with a 2-2 draw at West Brom, will fancy their chances at the Stadium of Light despite the fact United signed Argentina World Cup defender Marcos Rojo from Sporting Lisbon in midweek.
But with United having finished a lowly seventh in the table last season, former Old Trafford favourite Paul Scholes believes it will take more than one big-name signing to revive their fortunes.
"I am scared for United. Genuinely scared that they could go into the wilderness in the same way that Liverpool did in the 1990s."
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He added: "What do United need? Five players. Not five players with potential. Five experienced players... For now. Five proper players who can hit the ground running and turn around a situation that looks desperate.
"United's forwards are as good as any team in the league. The problem is what comes behind them."
The Belgian, signed by sacked United manager David Moyes following the pair's time together at Everton, has still to score a competitive goal for United.
Fellaini has joined a lengthy injury list that includes Michael Carrick, Jonny Evans, Danny Welbeck, Antonio Valencia, Luke Shaw and Tom Cleverley, all of whom missed the Swansea defeat.
But star striker Robin van Persie is nearing full fitness and could feature against Sunderland.
Chelsea will look to carry on from where they left off in 3-1 win away to Burnley when they face another newly-promoted side in Leicester at Stamford Bridge on Saturday.