The U's ended a run of six games without a win with a narrow victory over the Sky Blues, to move four points above the relegation zone.
Colchester took a fourth-minute lead when former Coventry loanee Freddie Sears latched on to a pass and confidently slotted past goalkeeper Joe Murphy.
The U's went further ahead midway through the first half when Gavin Massey finished low past Murphy from David Wright's long pass before Callum Wilson pulled one back for the visitors on the stroke of half-time.
Pressley said: "It's about concentration, without doubt when you defend and when you attack.
"As a group, we have to do better than that and you can't come away from home and find yourself 2-0 down after 20 minutes and expect to get something.
"Over the course of the other 70 minutes, when you take into consideration the pitch, which was appalling, I thought we played a lot of good football.
"In the end, I thought that we should have got something but we're making life far too difficult for ourselves and we have to be far more aggressive in our defending and be better with our concentration.
"But we looked a real threat on the counter-attack and at times, we looked like the team we were at the start of the season.
"There were a lot of good things to take but the first 20 minutes, we can't accept."