Lampard said after the match of the battle for the top-four with United: "They are a very good team, we know that. The individuals and the squad they have, the manager and coaches they have.
"I watched their game earlier and you can see the individual quality they have across their frontline. Two players are on 20 goals, Mason Greenwood is doing what he's doing scoring regularly."
Chelsea took the lead just before the half-hour mark when Ross Barkley played Olivier Giroud into the penalty area with a nice pass and the France international cooly slotted home with his first touch.
For the second match in a row, Christian Pulisic drew a penalty when Etienne Capoue recklessly clattered into him in the area and for the second match in a row, his teammate Willian stepped up and calmly dispatched it for a 2-0 lead at half-time.
Neither side could find the back of the net in the second half until Barkley buried a shot from right in front of goal in second-half stoppage time as Chelsea returned to fourth in the table on 57 points with five games left to play.
Watford remained fourth from bottom, a point above the relegation zone.
Chelsea, as well as relegation-threatened Watford, were under pressure and Lampard made key changes from the West Ham defeat, bringing in Giroud for Tammy Abraham, as well as Kurt Zouma and Reece James in defence.
But the home side laboured through the first half hour despite enjoying most possession against the bright yellow ranks of a well drilled Watford defence.
Giroud, then Willian twice, and Mason Mount smashed the ball into the defensive wall from usefully placed free kicks outside the area.
But Giroud's sharp strike relaxed the London side and rattled the visitors. The second goal came as no surprise with Pulisic's speed and movement a constant irritation to Watford. Capoue floored the American though he was running wide of goal and shook his head in disappointment when Willian calmly put away the penalty.
Giroud looked keen to add to his tally in the second half, forcing Craig Dawson to save with a well placed leg and Foster to smother another shot from close range.
Watford earning their first shot on target when substitute Adam Masina's distance effort was comfortably held by Kepa Arrizabalaga.
Masina had another crack from a free kick but again Kepa collected it before he had to dive low in the 81st minute to parry a shot from another Watford substitute, Danny Welbeck.
But it was Chelsea who capitalised on their 70% possession at the death, with a smart move involving Ruben Loftus-Cheek, Pulisic and Cesar Azpilicueta who crossed the ball for Barkley to ram home.
"We needed that one today," Pulisic told Sky Sports. "We came out with a lot of energy and we needed a huge result, especially after dropping points in the last game. There was a bit of extra energy because of that loss and we wanted to prove a point."
After the match Lampard gave an update on N'Golo Kante, who was substituted in the 78th minute with a hamstring issue.
Lampard said: "He got a small hamstring which will we have to scan tomorrow and see how bad it is. We hope it's a small problem, that's what I've been told so far. I'm guessing it'll be a problem for Palace on Tuesday."