Chelsea are the current holders after beating Bayern Munich on penalties in last season's final, but suffered defeat by Shakhtar Donetsk in midweek.
Premier League champions Manchester City are in danger of missing out on the knockout stages for the second year in a row after losing to Ajax, while Manchester United had to come from 2-0 down to beat Braga 3-2 at home.
United boss Ferguson does not necessarily believe the standard in England has dropped, but it is more that the level has improved across the continent.
"I don't think other countries are overtaking us but they're improving," said Ferguson. "We saw that with some surprises last season. Barcelona didn't reach the final, when they were hot favourites, and they've been lucky at home so far this time.
"Shakhtar Donetsk [against Chelsea] gave an excellent performance and Ajax [against Manchester City] also this midweek. Borussia Dortmund were unbelievably brave against Real Madrid.
"It's one thing criticising English teams, but you've got to recognise when their opponents play well.
"I don't think we needed a warning shot, but you always want to improve. The Champions League is not easy now. It's a difficult competition."