"It is very difficult to replace Ronaldo," Lampard said.
"I saw Alex Ferguson say that himself and I don't think he will look to replace him. He will look to make the team strong as a whole by not having him. That is the only thing they can do.
"The amount of times I flicked on the telly and saw Ronaldo had scored and they had won one or two-nil. Messi is probably the only player you could replace Ronaldo with.
"That is probably one of our big pluses this year - we didn't lose our big players."
Reflecting on the impact Ronaldo's transfer has had on the title race, Lampard added: "The league is probably closer now, which has been a bit of a natural thing over the past few years," he said. "In United's case, you don't want to draw the promoted teams so early in the season. They come out of the blocks to try and get some points. It [the defeat to Burnley] wasn't completely unexpected. I thought it might be a difficult game for them.
"We have now got a little bit of a lead over them and Liverpool. It is nice to get your noses in front."