Boulter was heard saying “everything was horrendous” to her team at the side of the court as she found herself trailing Bouzas Maneiro.
“A really tricky match for me. Today I was a little too passive and I didn’t quite play the way I wanted to play,” Boulter told BBC Radio 5 Live.
“These players are always going to have a go at you, you’re the higher-ranked person, they’re going to play better than their ranking.
“I have to be ready for that, and I have to be able to play my game better.”
The 28-year-old has leapt up the rankings after a strong 18 months, but she has yet to reach the second week of a Grand Slam.
Coming into the US Open, Boulter admitted she was not as prepared as she might have been for the hard courts, completing just three matches on the surface after playing at the Olympics on the Paris clay.
Meanwhile, Bouzas Maneiro was ranked outside the world’s top 150 at the start of the year and had played just one Grand Slam main draw match.
The 21-year-old has hit an impressive run of form, beating defending champion Marketa Vondrousova on the way to reaching the third round of Wimbledon before retiring injured against eventual winner Barbora Krejcikova.
She will play sixth seed Jessica Pegula in the third round after she beat fellow American and former Australian Open champion Sofia Kenin 7-6 (7-4) 6-3.
