Riley was handed his bow by Louis van Gaal when the new no.49 replaced fellow youngster Cameron Borthwick-Jackson to complete a full half of competitive action in United's Emirates FA Cup win over Shrewsbury Town.
"I'm very, very happy as this is something I have always dreamed of," Riley told MUTV. "Standing on the touchline waiting to come on, wearing the shirt with your name on the back, is something that every lad coming through the system is proud to do and it was the same for me – it's a dream come true."
Riley was an unused sub in the away defeat to Midtjylland last week, but was more than ready to answer his manager's call at Greenhous Meadow on Monday evening with the youngster clearly delighted to have been elevated to the senior squad.
"It [the past week] has been a great experience for me - out of this world," he added. “I'm learning so much from the players who are all top lads.
"My mum and my sister have always said that they would come to my first game, but they didn't travel to Denmark because it's a bit too far. They were here tonight though and it was a really proud moment for me and for my family too."