A first-half hat-trick by Leicester's Chris Wood condemned the Robins to their 16th loss of the season, with Matthew James adding the final goal in a 4-0 win at Ashton Gate.
Defeat sent City bottom and cost McInnes his job, and in a last press conference before his sacking was confirmed he said: "A lot of players let the fans down in the first half.
"The first three goals were self-inflicted. Dean Gerken would say himself that he didn't have the best of games, but there were errors in front of him as well.
"We were too pedestrian in our play and made it easy for a very good Leicester team."
McInnes, clearly in the dark about his fate, added: "I am looking to sign a few players before the transfer window closes."