Post Match Interview
"I think we were careless. It's a wake-up call in many ways. A lack of concentration saw us give the game away really...but we rescued it.
"To concede three goals at home...the defence and midfield have to be better than that in terms of concentration.
"If you lapse, the quality of the Champions League can make you suffer."
Ferguson was unimpressed by referee Paolo Tagliavento's decision to award Basel the 75th-minute spot-kick which gave them the lead.
The referee decided Antonio Valencia fouled Marco Streller - and Alexander Frei netted.
The Scot continued: "I don't think it was a penalty. You can see quite clearly that Valencia won the ball. It's a poor decision.
"The referee kept getting in the way of the game. I think he wanted to play. But what can you do?"
On two-goal Welbeck, Ferguson added: "He tired a bit in the second half but it was difficult to take him off...if we'd been in a better position we'd have probably taken him off."
Fabio joined United's injury list - however, the Scot is hopeful of having more options for the weekend's match against Norwich in the Barclays Premier League.
He added: "I don't think (Chris) Smalling will be back but hopeful of one or two others will be coming back."
Asked by Sky Sports if Wayne Rooney and Javier Hernandez were two of them, the Scot said: "Hopefully."