It will go down as one of the heaviest losses during the 2019 season for the defending champions.
"We always have to work very hard. You see the difference between the first half and the second half, it's a very big difference.
"We had maybe the first two big chances in the first half, but they were winning those second balls," he continued. "And three times they countered us: long ball, winning the second ball and they scored three times a goal. Also the penalty was the same story.
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"Second half we were winning those duels and we were much more attacking. Not that it resulted in very big chances, but we had the control. That's what we had to do from the start. We are very vulnerable if we don't win those duels. In the first 45 minutes that was the problem in my point of view."
Eastern Conference implications
The result meant that 2nd-place Atlanta United officially fell out of contention for the No. 1 seed in the Eastern Conference with two matches remaining: Sunday at Montreal (5 pm ET | MLS LIVE on ESPN+ in US; TVAS in Canada) and on Decision Day presented by AT&T against the New England Revolution.
"Of course, it's three points -- a very expensive three points to battle for first place," De Boer said about the defeat at Yankee Stadium. "But we have to be focused now for second place and to get home advantage, of course. We have to have to straighten our back, look forward and know what we were good at, show the second half was much better. And from there we have to continue."
Atlanta United are guaranteed to host a Round One match in the Audi 2019 MLS Cup Playoffs on the weekend of Oct. 19-20. If they meet NYCFC in the postseason, it would be in the Eastern Conference Final should both make it that far.