Ferguson joined his former side in the Spanish city in the hope of inspiring a second leg comeback.
But after a Lionel Messi brace inside 20 minutes, United were all-but out of the competition by half-time.
A Philippe Coutinho goal in the second period wrapped up the win for the home side.
But does Ferguson think Barcelona gave themselves an advantage ahead of the second leg?
The former United boss was spotted pacing from the touchline to the corner of the 18-yard box before the match, counting the number of steps.
“Sir Alex was out on the pitch for quite some time,” Gary Lineker said on BT Sport before kick-off.
“In fact he was pacing the ground at one stage.
“I’ve got a little bit of inside information because I was at the lunch with the Barcelona and Manchester United people today.
“They were arguing over which pitch is bigger.
“Sir Alex said this place, here, the Camp Nou was bigger than Old Trafford.
“Whereas David Gill and one or two others said ‘No, they’re the same size’. And he’s paced it out.”
BT reporter Des Kelly, who was beside the pitch, confirmed Ferguson was measuring the pitch.
And he claimed Ferguson thinks the pitch was widened for Tuesday’s match.
“Alex was pacing out the pitch,” Kelly said.
“He loves a conspiracy theory.
“He thinks it’s been widened. I’m not sure about that.”
Barcelona will face either Liverpool or Porto in the semi-final.