The Scotsman has his Villa squad in Philadelphia for a pre-season friendly with the Union at PPL Park and was asked at a press conference what he thinks about the state of MLS.
"I don't think the league should be underestimated, I really don't," he said.
"I know Scotland came over here a couple of months ago and were well beaten by USA national team (5-1).
"I don't think you can undersell it. There are some really good players in it and I think it's an improving league and we're in for a hard game, a really tough game."
As for Villa's chances against the Union, Lambert preferred to maintain a realistic outlook seeing as his side have only recently begun pre-season training.
"We are only one week into pre-season, we have had one game, still long way to go but we are improving," he added.
"The fitness levels are getting better. Playing a game tomorrow night is going to be really tough for us but we will do everything can to make it as competitive as we can. But we also know that Philadelphia have played a lot of games this season. It's something we are looking forward to."