The side remained unbeaten at home to reach last year's Champions League final which they lost to Barcelona, while dropping just two points in Manchester on the way to a record 19th English league title.
That is in marked contrast to this season, where they have won only one of five European home games and have lost twice and drawn once in the Premier League.
"I think there's been that slackness all season [with defending]," Ferguson told MUTV after his side were comprehensively outplayed by Athletic Bilbao in a 3-2 home defeat in Thursday's Europa League round-of-16 first leg.
The fact they are playing in the continent's second tier competition owes much to their poor home record in the Champions League group stage where they won only one of three Old Trafford games to fall short in what should have been an easy group.
"We've had a bad season in Europe, it has to be said," Ferguson said.
"We've lost three goals against Basel, two against Benfica, two to Ajax and three tonight. That tells the story. Maybe it's just one of these years when we need to take stock in terms of assessing how we should approach games at home."
They have conceded on average two goals per game at home in Europe this season and while the statistics are better in the domestic league, they are trailing in the table by two points to a Manchester City team with a perfect home record.
One of their Premier League defeats was the 6-1 thumping by neighbours City in October, which was an unthinkable scoreline for any team to inflict on United at Old Trafford, while the other home loss was to relegation-threatened Blackburn Rovers.
Ferguson's side return to Old Trafford on Sunday when they entertain mid-table West Bromwich Albion.
The manager will make a late decision on Chris Smalling's fitness after the defender aggravated a head injury during Thursday's game.
"His stitches came open in the first half," Ferguson said. "We thought we'd patched them up at half-time but they came open again."
The Scot added he might have to gamble on a couple of players against West Brom as the team faced the tough task of trying to overturn the Europa League deficit in Spain next Thursday.
"I'll make one or two changes. [Paul] Scholes is available and so are Nani and Rio Ferdinand to get the freshness back in," Ferguson said.
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