Five months after the highs of last year's grand final, the Swans suffered the embarrassment of losing to cross-town rivals GWS for the first time.
The Giants downed Longmire's charges by 24 points, while Carlton earlier inflicted a 22-point defeat in Sunday's triple-header at Blacktown.
"Probably the only real positive to come out of it was that we got some game time into some players," Longmire said.
The Giants and Blues are both about a month ahead of Sydney in their pre-season training, given finals football delayed the Swans' return to the track.
Sydney played both games consecutively in humid and wet conditions, while Longmire fielded inexperienced sides minus key premiership stars, including Adam Goodes and Josh Kennedy.
But Longmire said these were not excuses.
"Competitiveness, the contested ball. We thought we were a bit fumbly and we didn't take our chances with clean-ball handling," Longmire said.
"We didn't really have the composure that we wanted to."
Nonetheless, the third-year coach wasn't overly concerned about his premiership season prospects.
"We've still got time," he said.
"We've still got five weeks before the start of the season to get ourselves in the right condition and have all of our senior players back.
"But we still would have liked to be a bit more competitive than what we were today."
GWS coach Kevin Sheedy was thrilled with the upset win over Sydney, but disappointed his side snatched defeat from the jaws of victory against Carlton.
"We were very competitive tonight. We should have won the two games, that was really disappointing," Sheedy said.
"I know both the teams weren't the full squads, but neither was ours. We got a lot more better players we can bring back in."
The only injury concern from the three games was GWS defender Matt Buntine, who was assisted from the field after copping the brunt of Casboult's knee in a marking contest.
Buntine was sent to hospital for scans of suspected rib damage.
"He's sore. How sore I don't know," GWS coach Kevin Sheedy said.
"He won't play for a little while, he'll be out for a month at least."