Flew in earlier this week on United and the check in agent very specifically wanted to see a negative test. Also there was a "wall" of agents checking everyone at the Lisbon airport after getting off the plane (and before passport control or connection transfer point) and they also wanted to see our negative test results when they saw we had US passports.

So I definitely wouldn't chance it. Maybe you get lucky and don't need it, but that was not the case at all for us. We used a rapid PCR from Curative.