Lacey, 18, was brought on just two minutes before Danny Welbeck doubled Brighton’s lead at Old Trafford in their FA Cup third round tie.
Shortly after Benjamin Sesko pulled on back in the 85th minute, Lacey was shown a yellow card for his tackle on Ferdi Kadioglu.
Clearly frustrated by the decision, the youngster picked the ball up and threw it to the ground, which the referee deemed to be worthy of a second yellow.
Speaking after the game, Fletcher called the decision to give Lacey his marching orders ‘ridiculous.’
Fletcher told TNT Sports afterwards: “Shea Lacey’s yellow card is ridiculous compared to the fouls that we had against us all game, which ultimately then the lad makes a mistake and is sent off. For me it is poor refereeing.
“He's disappointed because he cares and he understands and he knows he will learn from it. There is no doubt about his talent. It was a steep learning curve today but we believe in Shea.
“He's one of our most exciting young prospects and I'm sure at Old Trafford he will have a chance to make amends for that in the future.”