Smith said: "It was amazing that the fans were still backing us when we were three goals down. We certainly wouldn't get away with a performance like that at Elland Road.
"Their support has been magnificent and they deserve better than what we gave them on Monday.
"All I can do is promise them that what took place at Leicester will not happen again.
"There were no excuses from anybody. We let ourselves down, we let the fans down and we let the manager down. All the lads know there was not a single good performance out there.
"We could have played until next week and not scored. We have to be better against Birmingham on Saturday and put on a show."