The 20-year-old Spaniard was at fault for West Bromwich Albion's goal in the champions' 2-1 win at The Hawthorns, a blunder which came a week after his shaky performance in the Community Shield was widely criticised in the newspapers.
"David should have done better but I think it's just a bit of concentration there... welcome to English football," Ferguson told Sky Sports after their opening league win.
"My recollection of goalkeepers coming from Europe is that it's a learning process, it's a different game."
He added that De Gea, who arrived at Old Trafford from La Liga side Atletico Madrid for 18 million pounds, need only remember United great Peter Schmeichel's wobbly start at the club to be sure things would get better.
"But Peter came on to be the greatest goalkeeper of all time possibly," added Ferguson.
Last Sunday, the Spaniard had been stranded when Joleon Lescott headed Manchester City in front in the Community Shield at Wembley before appearing to lose his balance and letting in Edin Dzeko's long-range effort.
Asked how he would deal with De Gea, Ferguson - better known for his 'hairdryer' treatment than a softly softly approach - replied: "Pat him on the head... and say to him 'welcome to England'."
