"He was a bit unlucky," said Ferguson, "and maybe [the penalty] come too early."
The United manager was overall pleased with his team's play on the road.
"It was a professional performance," he said, "and as I said at halftime we sealed off areas where Barcelona normally do well. There was a lot of play on the edge of our box but we defended well. We should have done better with our possession and we gave the ball away too easily. The result gives us a good chance."
United was awarded a penalty when Ronaldo headed Wayne Rooney's corner against the arm of Gabriel Milito.
Ronaldo thought he should have been awarded a second penalty later in the first half when he collided with defender Rafael Marquez.
Ferguson wasn't so sure.
"You're lucky to get one penalty at the Nou Camp," he said, "so getting two..."
"There were no white hankies, which showed that they were satisfied with the way their team played," he said. "Barcelona have fantastic players and I'm pleased that, while they kept the ball really well, they did not create that many clear chances. There was a period in the second half when [Samuel] Eto'o hit the side netting and got to the byline but otherwise we kept them at bay. Rio Ferdinand played fantastically."
Barca played without Ronaldinho, Lionel Messi came off after an hour, and Thierry Henry only played the last quarter of an hour. Still, Barca coach Frank Rijkaard was confident that his team will do well in the second leg on Tuesday at Old Trafford.
"We have to keep an eye on [Lionel] Messi still and take care with him," said Rijkaard, "while [Thierry] Henry is only just coming back from a fever, but we are getting key players back. I think that, while this is not a good result, we have taken a big step forward tonight."
April 23 in Barcelona
Barcelona 0 Man. United 0.
Barcelona -- Valdes, Zambrotta, Marquez, Milito, Abidal, Xavi, Toure, Deco (Henry, 77), Messi (Krkic, 62), Eto'o, Iniesta.
Man. United -- van der Sar, Hargreaves, Ferdinand, Brown, Evra, Cristiano Ronaldo, Carrick, Scholes, Park, Tevez (Giggs, 85), Rooney (Nani, 76).
Referee: Massimo Busacca (Switzerland).