The midfielder has played a right-back in all three of the Gunners' Premier League games so far, as they made an unbeaten start to the season, with Gabriel Magalhaes being left on the bench to the surprise of many.
But, according to the Ghanian FA, Partey has been ruled out for "a while" leaving Mikel Arteta to tinker with his defence.
A statement read: “Arsenal ace Thomas Partey is left out of the squad due to injury.
“According to his club, the player picked up an injury in training and could be on the sidelines for a while, however the Black Stars medical teams is monitoring the situation."
Arteta could now turn to Gabriel for the visit of United on Sunday, playing him at centre-back alongside William Saliba like much of last season with Ben White moving to right-back.
Takehiro Tomiyasu replaced Partey during the second half of Arsenal's 2-2 draw with Fulham, so is another option for Arteta.