The team shirts also bore the message: "One name, too many" at the bottom in solidarity with the Black Lives Matter social justice movement.
The Union players walked out wearing "Black Lives Matter" T-shirts, then proceeded to take them off for the team photo, turn around and reveal that their surnames on the back of their shirts had been replaced by victims of police violence, including (George) Floyd, (Breonna) Taylor, (John) Crawford, (Eric) Garner, (Alton) Sterling and (Tamir) Rice.
"Our players stand together," read a tweet by the team. "One name too many."
During the match, which took place inside ESPN's Wide World of Sports Complex, Union players also wore shin guardsthat read "enough is enough," with the hashtag "end racism" and raised fists pictured.
Union goalkeeper Andre Blake had the last name of George Floyd on the back of his shirt.
Union manager Jim Curtin expressed his support for the players.
"At the start, certainly things going on in our country far bigger than soccer," he said after the match, which Philadelphia won 1-0. "I have to say I'm very proud of my players throughout the past four months for the leadership they've shown, the leadership role they've taken in the BLM movement, educating the other players on our team.
"The idea today was action over permission; I hope the league understands that. It was done to show respect, to learn, to grow, to make our country better. I'm really proud of my players for everything they've done."
Union defender Ray Gaddis, whose shirt had the last name of Breonna Taylor, said the idea came from talking to all of the team's players.
"It was to continue the conversation that needs to be had. We first and foremost asked our team if they were OK with it because solidarity is key and we want to make sure everybody feels comfortable," Gaddis said after the match. "Actions are louder than words. Again, it's to further the conversation and to continue to use our platform to be a voice for the voiceless. It was a collective group effort."
Gaddis said it was important to see the entire team be on board with social change.
"For me, being a player in the Philadelphia Union organization for a while now, it means a lot," he said. "For me, within our club, it shows how much solidarity we have -- on the field first and foremost, but off the field. We have a great coach who has taken a stance with us, as well as this organization, and it's only going to make us better in the future, not just as soccer players but as human beings as well."
On Wednesday, more than 100 Black MLS players raised their right fists and took a knee before the tournament opener in a similar show of solidarity.
At that match, some players wore T-shirts over their jerseys with the words "Black and Proud" and "Silence is Violence" before the match between Orlando City SC and Inter Miami CF, and both teams' starting players, as well as the referees, took a knee before kickoff.
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Last month, more than 170 Black MLS players formed the Black Players for Change organization, which has three goals: have a voice in all racial matters as they relate to MLS, increase Black representation in the MLS Players Association and the highest levels of MLS, and have an impact in Black communities.
The kneeling protest was popularized by former San Francisco 49ers quarterback Colin Kaepernick in 2016, and the fist raising mirrors the protest against racial inequality by Americans John Carlos and Tommie Smith at the 1968 Mexico Olympics and came to be associated with the Black Power movement of the 1960s and '70s as well as the current Black Lives Matter movement.
Kaepernick's protest came during the playing of the national anthem, which will not be played during this tournament, given that the games are being staged in empty stadiums because of the coronavirus pandemic.
The Union's game against NYCFC is the second of the MLS is Back Tournament. Orlando City defeated Inter Miami 2-1 in the opening game Wednesday.
The competition, which is being held inside a biosecure "bubble" at the ESPN Wide World of Sports Complex, makes MLS the first major North American team sport to return to action since the COVID-19 pandemic upended the sports calendar in mid-March.
Philadelphia Union reveals jerseys honoring Black victims of police brutality
Players from Philadelphia Union are wearing names of Black victims of police brutality on their jerseys.
How MLS's powerful act of solidarity came about
Ray Gaddis, Justin Morrow and Earl Edwards Jr. reflect on MLS's act of unity in support of the BLM movement.