The Premier League champions cruised to victory in their opener at Swansea, but then stumbled to a home draw against Chelsea before losing tamely at Liverpool on Sunday.
Some fans and the press have also lambasted the club's seemingly chaotic tactics on transfer deadline day, with the failure to land Athletic Bilbao's Ander Herrera and Real Madrid's Fabio Coentrao.
The arrival of Everton's Marouane Fellaini silenced some of the doubters and an upbeat Ferdinand told the London Evening Standard: "We've got a strong enough squad to win everything.
"We won the league at a canter last season and we've not become a bad team overnight. We believe strongly in what we have got. And anyway, you can't judge the league after three games."
United boss David Moyes will be desperate to shine on the European stage this season with the Scot set to taste his first experience of a Champions League group game against Bayer Leverkusen at Old Trafford.
One player who will not be featuring in the competition until at least Christmas is midfielder Darren Fletcher who continues to struggle with a chronic bowel condition despite having another operation earlier in the summer.
The Champions League squad United named on Wednesday included youngsters Sam Johnstone, Tom Thorpe, Larnell Cole, Jesse Lingard and Will Keane, alongside Fellaini.
Cole and Keane are both 18 with Johnstone, Thorpe and Lingard aged 20 and their names were among those submitted to UEFA even though they all qualify for the B list because of their age, meaning they could still be promoted to the first team before midnight on the day before any Champions League fixture anyway.
Fellaini and Wilfried Zaha, in his first season at Old Trafford after signing in January but returning to Crystal Palace on loan, are set for their first Champions League action with the Red Devils.