United went seven points clear of Manchester City by coming from behind to claim all three points for the ninth time this season and the second against Saints as Wayne Rooney scored twice after Jay Rodriguez had put the visitors ahead.
But United had to hold on in a second half where Mauricio Pochettino's team applied pressure and Ferguson felt the Old Trafford pitch was the reason his team failed to pass the ball well.
"If you look at it my experience of this club and going for championships, there's always a game where you say we were a bit lucky and I think it was one of these nights," he said.
"I thought Southampton in the second half was the best performance anyone has played here this season. They pushed right on top of us and didn't give us time to settle on the ball. Second half, we just found the pitch impossible. Once the pitch dried up, the players found it difficult."
Rooney's brace took his tally to nine goals in his last eight starts and Ferguson added: "If Chicharito [Javier Hernandez] and Wayne and Robin [van Persie] get us 20 goals each we are in business."
Van Persie has 22 goals for the season while Hernandez is on 14 and Rooney is now up to 12.