Leeds were good value for the points as United saw defender Lisandro Martinez for pulling the hair of striker Dominic Calvert-Lewin. The red card came after Leeds took a 2-0 first-half lead through a Noah Okafor brace.
Fernandes said afterwards, "We gave the game away in the first half. We were not up to the game straight away with our intensity. We knew they wanted to press as 1v1, we had to be better on the first ball but even better on the second ball.
"We weren't great, we gave too much space away. We ended up going 2-0 down, we had time in the second half to turn it around but it was not possible."
Monday night was United's first game in 24 days, but Fernandes insists are were no excuses for the result.
He continued: "It is what it is. We are out of the FA Cup so we have to blame ourselves to be out of the game for so long. We had to prepare ourselves in the best way. We did but it wasn't enough to win the game."
On Lisandro's red card, Fernandes added: "I'm not talking about the referee. If I talk about the referee I'm going to get in very big trouble because the rules are applied differently for everyone. You can see it also in the yellow cards. It's better that I don't say anything."