City won the first-leg of the tie 9-0.
"They're professional - they never ease off," Clough said. "They don't just beat you, they annihilate you.
"I don't think we could've done too much more with the quality that is out there. We could do better with one or two but the gulf of the two teams is far too much to comprehend.
"We'll probably never be here again and that is how big this achievement is. A lot of clubs have never reached one."