Ferguson - who saw his side equalise twice at 1-0 and 2-1 down before losing 5-2 - had Lewis dismissed for the foul on Morten Gamst Pedersen which resulted in Steven Reid scoring a penalty.
"It's disappointing for the fans and it's disappointing for everyone," said Ferguson.
"It's not as if we came here and got annihilated. Obviously as the game went on, they kept the ball and I started to become more concerned about us keeping some energy for Saturday.
"But we gave them a game up to the sending off and that was the turning point. I would have to see it again - I have only seen it once live.
"He (Pedersen) made his decision and Joe tried to get out of the way of him, you can see in his body shape. But he (Pedersen) has just nicked it around him, kicked it out of play and the ref has sent him (Lewis) off and given a penalty."