The 19 year-old had only taken part in his first training session on Friday and was surprised to be handed his debut at Old Trafford.
“It was a surreal experience," Colwill told the club's website. “I wasn't expecting to come on because I had just arrived, I wouldn't have put me on if I was the manager! But I have a lot of respect for the gaffer for putting me on in an important moment.
“When I did go on, it didn't feel real, I felt like I was dreaming. I headed the ball straight away and it woke me up, I thought 'I've got to play now'. It was brilliant. I didn't have time to think about it, that was the good thing.
"I was straight in the deep end, but we came out the other side with a win and that's all that matters. I am happy."
Now the defender wants to build on his debut.
“Making history in my first game, I can't complain! Hopefully we can do some more good things this season."