"I have played close on 900 games and this is one of the most difficult matches I have been in," he said after the 0-0 draw at Highfield Road.
"It lacked quality. We couldn't get the ball down and play it and in the end it was very, very dour.
"I looked around and saw guys that were very sharp and lively on Saturday, looking extremely leggy and tired.
"Maybe the energy and atmosphere of Saturday took its toll and it was certainly a case of after the Lord Mayor's Show.
"Our level of consistency is just not there. At the moment you find as you step down from the Premier League and down further into the first division the ability of players to repeat good performances decreases."