Westwood has become a regular feature in Lambert's starting XI this season after joining from League One outfit Crewe Alexandra last summer.
Lambert believes that Westwood performs a vital role for his side which often goes without credit from fans and analysts.
He told Sky Sports News: "I call him [the water carrier] but it's the biggest compliment I can pay him. He's brilliant at it. He does the hardest thing well, which is just keeping the ball.
"The thing is, he never gives the ball away. I know exactly what I'm going to get with him. Coming from where he was and to do what he has done here is absolutely phenomenal.
"Players like Christian Benteke get all the headlines and I suppose a lot of the work Ashley does go unnoticed by some. But not by me. The team functions when he's getting the ball and giving it. He's been great."
The 20-year-old has made 29 appearances so far in his first Premier League campaign at Villa Park.