There's still plenty of competition on both sides of central defense, though, as the Americans continue to prepare for next week’s tournament.
“It's strong. We've got a bunch of good guys at that position,” Birnbaum told reporters on Friday. “Everyone's fighting every day to play. Everyone's excited to keep the competition going.”
The competition on the right side just got tougher, too, with veteran Geoff Cameron – who has been nursing a hamstring strain – available for Saturday's final tune-up friendly against Bolivia (8 pm ET, FS1).
“In the center back situation, we are very blessed,” coach Jurgen Klinsmann told reporters ahead of Friday morning's training session. “We have guys there that are ready to make that international presence now that is needed in Copa America. John Brooks made a big step forward over the last year. Michael Orozco has always been very consistent with us. Steve came into that group. Matt Besler is there. He's playing, so far, a very strong season as well.
“So we are very blessed, having Cameron come back now from that little issue, so it's very, very tight. It's not easy for us coaches always saying, ‘This guy has the edge’ or ‘That guy has the edge,’ but it’s our job at the end. You can only field 11 guys.”
Birnbaum, who made his US debut in the 2015 January camp and scored against Iceland this past January, started next to Brooks – who is battling Besler for the starter's spot on the left side – and went the full 90 in Wednesday's 1-0 victory over Ecuador in Frisco, Texas.
“I thought it was a little shaky in the beginning,” Birnbaum said. “Maybe it was nerves, or whatever. I couldn't get a rhythm. But I thought I settled in once the game settled down, and keeping it at zeroes was all I really cared about. That's the biggest thing.”
Klinsmann liked Birnbaum's work rate against Ecuador, even if the execution took a while to come together.
“It's important, these two games – maybe including Puerto Rico – that a couple of guys, we see them compete here in that group, and we see where they are right now,” Klinsmann said. “Steve competes very well, and that's why he's here. That's why he deserves to have a spot in these 23.”