The 22-year-old enjoyed an impressive outing as Vicky Jepson's side fought back from a goal down to beat Casey Stoney's team in the Continental Cup group stage on Wednesday afternoon.
Rachel Furness netted twice for the Reds, including a penalty, and Rinsola Babajide also put away a spot-kick.
Canada U20 international Foster joined the club in January 2020 from West Virginia University - and for the lifelong Liverpool fan, her dream is now a reality.
"I honestly can't put it into words, it's really emotional for me and I know my family back home in Canada are emotional too," she told Liverpoofc.com.
"It was a great performance by the girls and I couldn't have asked for a better debut to get the three points.
"Overall, I was pleased with my performance and the team's performance and we can't wait to go again at the weekend.
"We can take so much confidence from this game. To beat a team from the WSL who have held their own against the likes of Chelsea is just amazing for us and it just goes to show we can compete with the players in the top league.
"We will go to Leicester on Sunday with a lot of confidence but also staying humble and we'll keep on attacking every challenge."
- Match report: LFC Women 3-1 Manchester United Women
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Foster was called into action just two minutes into her debut, making an important save to deny USA international Tobin Heath.
"It was just a great way to get myself warmed up and ready for the match!" she reflected with a smile.
"You never know what is going to happen in the first five minutes of a game, so it definitely put me on my toes.
"No minute is going to be an easy minute so I was just really pleased to be strong and keep it going throughout the whole game."
At the other end, Northern Ireland international Furness continued her rich vein of scoring form in 2020-21.
The attacking midfielder's double against United took her tally for the season to six goals.
"I can't put it into words how good she is for us," said Foster.
"Her graft, her passion, her heart on the field, it doesn't go unnoticed and for her to score a penalty and letting Rinsola take the second one just shows how much of a leader she is and how important she is to this team."
LFCTV GO: Watch LFC Women v Man United highlights
Foster has been learning her trade this season alongside summer signing and No.1 Rachael Laws.
The Ontario native firmly believes she can become a better goalkeeper working every day with Laws and goalkeeping coach Jen Herst.
"Lawsy is just a great leader and a great mentor," she said.
"The way she trains, the passion she brings every day, her comments on the field and just her presence, she is one of the best goalkeepers I've had the pleasure to be around.
"Learning from her is really important to me. She has taught me a lot and putting it on the field today against United, I guess she'll be pretty proud of that."