Chelsea went into Sunday's clash at Old Trafford four points ahead of Tottenham with a game in hand, but squandered the chance to restore their seven-point advantage.
Marcus Rashford and Ander Herrera scored either side of half-time to consign Antonio Conte's Blues to their fifth defeat of the season, with Spurs within two results of top spots as the London rivals battle for the trophy.
Chelsea's all-time top goalscorer Lampard won three league titles at Stamford Bridge, and said Conte's men must forget their latest setback as they did the 3-0 defeat at Arsenal in September, with the Blues bouncing back from that loss to win 13 games in a row.
Speaking to Sky Sports, Lampard said: "They weren't there today at all, the Chelsea that we know.
"Credit to Manchester United, they came out the blocks and I think they were first to every ball.
"Everything seemed to drop for Manchester United players. That wasn't luck - that was because they were sharper than Chelsea players.
"The only thing I can say for Chelsea is to just wipe it out of their memory, it's one of those games.
"They've been fantastic, they've been winning games regularly. They played Arsenal earlier on in the season and they kicked on from that with a similar kind of performance.
"I think today there's no way they can dwell on that now because they've got an important run-in and now it's game on, because it's four points."
Chelsea suffered another defeat recently against Crystal Palace, and Lampard added: "It's tough because losing games brings an element of doubt, you can see that even in their faces walking off the pitch, the manager's thinking and they're all a bit upset, it'll be a big week for them now,
"It needs characters in the team and I think they are there but it needs the big characters in the team to settle the ship, work hard in the week, get ready for their next games and put in the performances that they have put in all season."