According Robin Walker, who coached Pulisic at non-League side Brackley Town in 2005, the Chelsea attacker never contemplated going to Old Trafford because of their former coach.
Mourinho's poor track record of giving youth a chance saw Pulisic's father all but block the potential move with the 20-year-old instead signing for the Blues in January with Walker's encouragement.
"He wouldn't go to Manchester United because of Jose Mourinho," Walker told The Telegraph. "His father couldn't stand Mourinho because he didn't promote young players.
"It was at that point I asked, 'What about London? That's where it's at.'
"I'm a Chelsea fan, I grew up in London supporting them. When I was in Dortmund last year, I told him that he had to come."
Signing for Chelsea at the start of the year before being loaned straight back to Borussia Dortmund, Pulisic has started the Premier League season at Stamford Bridge and the early signs are encouraging.
After starting and claiming an assist in the Blues UEFA Super Cup loss to Liverpool, the American winger played 71 minutes against Leicester City on the weekend with his coach impressed with what's he's been seeing.
“I am really happy with him," Lampard said last week.
"There is lots, lots more to come. Again, [he’s showing] all the good signs for what he can be for the club.
"The expectancy is right, he is a big signing for us but you have to remember his age."
While Lampard is looking to keep Pulisic grounded, former Blues striker Tony Cascarino recently declared the attacker could one day match Eden Hazard's quality.
“Christian Pulisic looks like he will be a very dangerous player for Chelsea," Cascarino told The Times.
“I don’t remember a young Eden Hazard being as quick and direct and there’s no reason Pulisic can’t develop into a player of Hazard’s standing.
“Pulisic reminds me of Arjen Robben, you cannot stop him one-on-one because you don’t know which way he’s going to go and his pace is devastating.
“He will be a massive asset for Frank Lampard this season.”