The striker has been on the fringes of the side for much of the season, but with several forwards ruled out through injury, suspension and unavailability in Tuesday’s Johnstone’s Paint Trophy tie, was handed a second career start.
Marsh, who turned 19 on Christmas Eve, seized the opportunity and took 14 minutes to open his account, heading home in the 3-3 draw with Southend United, before Oxford lost 5-3 in a penalty shoot-out.
Lewis was pleased with the overall performance of the youth team graduate against a strong Southend defence.
The United assistant manager said: “Tyrone’s done really well. He’s a young lad with a lot to learn still, but I think he’s done himself justice against two centre halves who have a lot of league experience.
“At times he gave them a bit of a torrid time.”
Marsh has had to be patient, with Tuesday’s game his first at the Kassam Stadium.
And despite getting off the mark it is far from certain he will stay in the starting XI, with more experienced striking options available for Saturday’s npower League Two trip to Bradford City.
He added: “The goal will give him a bit more confidence and it shows he can do it. “As with all young players, consistency is the key.
“We see in training that he has the ability to produce it, but the major key for him to move on is doing it day-in day-out.
“He wouldn’t have been playing if we didn’t think he could produce a performance.”
One player to miss Tuesday’s game was Peter Leven, who had a sore knee after Saturday’s FA Cup defeat by Sheffield United.
Lewis is optimistic the influential midfielder will be in contention for this weekend.
He said: “Peter Leven wasn’t quite right and it wouldn’t have been fair to him or ourselves with a game coming on Saturday.
“There’s no guarantee that he’ll be fit for Saturday, but I’m saying that because he didn’t play it will give him every chance.”