Richie Barker admitted his Crawley players were below par following their 1-1 draw with lowly Colchester.
Anthony Wordsworth's late penalty denied the Red Devils victory after Kyle McFadzean's controversial goal had put the visitors in charge.
McFadzean appeared to handle the ball before putting Crawley ahead with 19 minutes to go, while Mike Jones twice hit the post for the visitors.
Barker said: "I thought we played within ourselves in the first half. I said to the players at half-time that I thought we played in second gear.
"I don't know why; perhaps it was a bit of tiredness or the fact that we had had an excellent away win at Preston in front of a big crowd last weekend. Perhaps it was because Colchester - with all due respect to them - were bottom of the table.
"Whatever it was, they found it difficult to lift themselves. That was disappointing but it was better in the second half. We hit the woodwork and sometimes, it's just not your night.
"I didn't have a good view of either goal but my players didn't agree with the penalty decision. Their players were upset with the manner of our goal as well I think, so you could argue that they're both slightly controversial."
Colchester have now gone seven games without a league win but ended a run of three successive defeats, in front of their lowest-ever league crowd at the Community Stadium.
Colchester manager John Ward said: "We've probably played better in the two defeats we've just had but that's the game of football. We've had a decision go against us but that's how it goes.
"We went very positive in the second half, and got Anthony Wordsworth on the ball more. We got something out of the game, against a team that is fourth in our division and had just won at Preston which was a fantastic result for them."