The Uruguayan had to be withdrawn after quarter of an hour following a challenge from James Bolton.
"He was in a lot of pain. He is in a brace at the moment, he will be assessed in the next few days, and we will know more then.
"I don't know if it's the bone or ligaments, we don't know yet."
Mark Mann-Bryans of the Press Association shared footage of Torreira leaving Fratton Park on crutches and wearing a protective boot:
The 24-year-old started just twice in Arsenal's first 10 Premier League matches this season.
Unai Emery began using him more in the final weeks of his reign in November, and he retained his place as a regular starter under interim boss Freddie Ljungberg and then when Arteta took charge in December. Though this was his first start for the Gunners since their 2-2 draw with Chelsea on January 21.
Torreira is a tenacious presence for Arsenal in midfield, and he works hard to break up opponents' play, giving the Gunners intensity and bite.
He excels in protecting the ball to draw fouls from opponents, too—only Matteo Guendouzi has drawn more per game than Torreira for Arsenal in the Premier League this season, per WhoScored.com.
Arsenal advanced to the quarter-finals of the FA Cup on Monday courtesy of goals from Sokratis Papastathopoulos and Eddie Nketiah either side of half-time:
The competition has been their biggest source of success in recent years, having lifted the trophy in 2014, 2015 and 2017.