Bebe made his United debut in last night's Carling Cup win at Scunthorpe United.
"It was one of those instinctive things, you have a smell about something and you take it," Ferguson told CNN.
"We have a scout in Portugal, where we asked him about this young player playing in the second division in Portugal. He'd only played three or four games and our scouts insisted that we should sign him.
"It was a bit of a leap into the dark really, because we hadn't seen him play.
"We sent one our scouts down... to see him play in the last game before we signed him and then he comes up and says there's something exceptional here, we're maybe seeing something really special, and so we took the gamble.
"So he's joined us but physically, he wasn't ready, hasn't done a proper pre-season, so the last few weeks since he's joined, we've been working on him physically."