Jonathan dos Santos was forced out of the LA Galaxy's 3-1 loss to the Columbus Crew with a left hamstring injury, though the substitution appeared to be more of a precaution than anything for the midfielder.
The news will still be somewhat troubling for Mexico, with reports surfacing earlier in the day that Hirving Lozano would miss out on this summer's tournament and reports also claiming that both Javier "Chicharito" Hernandez and Hector Herrera could miss out as well.
Long was considered a potential starter for the USA in this summer's competition but will now face a race against time to be fit enough to play in the competition.
The 2018 MLS Defender of the Year had scored New York’s opener with a fine header before going down clutching his hamstring late on, leaving the Red Bulls with 10 men as the club had already used all three subs.
"It could be a slight strain in his hamstring," Red Bulls head coach Chris Armas told reporters after the match. "That's all we know right now. There are steps that will be taken to see exactly what's going on there."
With DeAndre Yedlin having undergone sports hernia surgery on Tuesday that will force him out of the Gold Cup, Long's injury could leave Gregg Berhalter weighing options on the back-line without two potential starters heading into the June tournament.
Both teams still have over a month before opening group play, with Mexico kicking off against Cuba on June 15 and the USA set to open against Guyana three days later.
Mexico headlines Group A, with Canada and Martinique joining El Tri and Cuba, while the USA and Guyana will be joined in Group D by Panama and Trinidad and Tobago.
The Gold Cup will also be the first tournament in charge for both Berhalter and Mexico boss Gerardo "Tata" Martino.