Ipswich boss Lambert claimed the only reasons Sancho should move to United were "football or family" and insisted the atmosphere at Dortmund is superior to Old Trafford's.
"Sancho is already at one of the biggest clubs in the world, which gets over 80,000 for every league game," Lambert said.
“So, potentially, the lad has a big decision to make. Dortmund is a special club and you only realise it once you leave.
“United are one of the biggest clubs in the world but is the atmosphere at Old Trafford as good as the Westfalenstadion? The answer is no.
“He will have to ask himself will he fit in with the right players if he leaves. Will he keep making progress? Dortmund will be in the Champions League but we are not yet sure about United and whether they will be.
“If he comes home, it should be for football or family reasons, not money."