The club formed the team in 2018, and Casey Stoney's side won promotion to the Women's Super League in their first season. United sit at the top of the WSL table after six matches.
"I think women's football in England is the same as in America -- it is so far behind because of what we've had to overcome in the lack of investment," she told the BBC in a story posted on Monday.
"It's 2020. How long has the Premier League been around? And we're only just seeing a club like Manchester United put effort and pounds towards a women's team? Frankly, it's disgraceful."
Rapinoe also said she didn't feel supported by the U.S. Soccer Federation or then-national team coach Jill Ellis for taking a knee four years ago.
The two-time World Cup winner protested during the national anthem in 2016 in support of Colin Kaepernick against racial injustice and police brutality.
"Taking a knee sharpened my understanding of what I did, how I supported Colin and sharpened my resolved in that," she said.
"It was not a comfortable time. I certainly felt that at least my international career was a little bit in jeopardy. It showed me a lot of true colours around me in a lot of different ways.
"The critics yell really loud, and I had a federation who clearly didn't support me and, I felt, a coach who really didn't support me, but I had so many people around me who always stuck by me and were with me the whole time.
"But you have a choice of what you do in the world. You just have to be prepared to wear the consequences of your actions."