Millwall manager Kenny Jackett was left frustrated after his side's play-off hopes were dented in a 2-1 defeat at Coventry.
The Lions were without the suspended Steve Morison and his injured strike partner Kevin Lisbie, and were left even more short in attack when Neil Harris was sent off.
Andros Townsend was on target but two Marlon King goals secured victory for the Sky Blues.
Jackett said: "Our front two were not playing for different reasons, we lost our captain Paul Robinson in the warm-up and I think it perhaps showed in the start.
"We were quite tentative in our play, which is not like us. We gave a bad goal away that put us 1-0 down. We clawed our way back into the game with 11. Even with 10 we did very well.
"The players put on a good performance but right now it is a sense of frustration for me because it is an opportunity where we were unlucky to get nothing out of the game whatsoever."