"It was tough. Firstly conceding a set piece was not the best, we tried to push and maybe overexposed ourselves, played some passes we shouldn't and they hit us on the counter, and got the second goal and the third goal hit home and hurt. Second half we weren't good enough and we have a lot to work on in the international break," Colwill told Sky Sports.
The defeat extended a troubling pattern for Chelsea at set pieces, with Colwill acknowledging the issue has become a recurring weakness.
"Not been good enough. At times we have been bullied in the box, we need to definitely improve with that. We have got all trust in our set piece coach and still adjusting to new things.
"We do need to improve as a team," he said, having now been part of a defence that has conceded five goals from set-piece situations already this season.
Despite the scale of the defeat, Colwill insisted the dressing room's morale had not suffered as a result.
"I don't think spirit went down. It hurt and we tried to push for equaliser and we got hit on the counter attack. I don't think our spirit dropped," he said.
He also pushed back against relying too heavily on Chelsea's front line to paper over defensive shortcomings.
"We still need to improve a lot. We can't rely on our front three to pull us out of danger. The rest of us have to help as much as possible. We definitely have a lot of room to improve."
The defeat leaves Chelsea to reflect on a difficult afternoon during the international break, with head coach Xabi Alonso having already identified defensive consistency as a priority since taking charge in the summer.
Colwill's candid assessment suggests the issues run deeper than a single afternoon, with the club's vulnerability at set pieces in particular now a recurring theme that will need addressing before the season's fixture list intensifies further.