Jaidon Anthony hit Burnley's equaliser on 66 minutes after United had jumped 2-1 ahead through a Benjamin Sesko double.
Fletcher said at the final whistle: "For me, the fact we have drawn is a massive disappointment. If you look at the context of the game, the chances we created, goalline clearances, a disallowed goal, 30 shots, lots of attacking play. We created more than enough to win the game so obviously it is disappointing when you only come away with a point.
On their improvements after halftime, Fletcher explained: "I challenged the players to be better. I felt we were sloppy, I felt we improved as the half went on but I didn't think we started great. I thought our energy could have been more, the tempo in our passing and play could have been more. More forwards runs, and the players responded.
"I thought they were fantastic second half, we created so many opportunities and I'm so happy for Ben with his two goals. We should have scored more, that's something we're going to have to work on, and we conceded two goals as well, so that's a disappointment."
On Sesko, he added: "I was so happy for him. I spoke to him yesterday and we presented a video showing his movement and showing his goals, just showing how it is coming and he needs to keep believing and the players need to get more service to him. He did that tonight and scored two great goals. A real positive night for Ben."