The German midfielder, who joined Villa on a free transfer from Bayern Munich last month after his contract expired, revealed on social media that his body has struggled to adapt to full training loads following an extended period without competitive football.
"Unfortunately, my body has reacted to the increased workload after my time away from the pitch," Goretzka wrote. "It's frustrating, but I need this short break now to come back at full strength. I'll use the coming days, including the international break, to recover fully."
Villa begin their Champions League campaign away to Club Brugge on Tuesday without six of the players who helped them beat SC Freiburg 3-0 in Istanbul to win the Europa League last May. Emi Martínez, Lucas Digne, Ezri Konsa, Youri Tielemans, Morgan Rogers and Ollie Watkins have all left the club since that triumph, part of a summer overhaul that has reshaped Unai Emery's squad. Goretzka, a 73-cap Germany international who won 17 trophies during seven years at Bayern, was seen as a significant free addition to help offset those departures, but he has yet to make a single appearance since signing.
Villa have also endured a difficult start domestically, going into the international window without a win or a goal in the Premier League this season. Emery is hoping a return to European competition can help shift the team's fortunes, though he was pragmatic about losing Goretzka before his debut.
"Goretzka's issue is a bit frustrating, but we have players who can replace him," Emery said. "When he's coming back, hopefully he can get the consistency in training and playing and we can add his experience. He is a player who will help us in different competitions with the level he has."
The timing compounds an already stretched midfield picture at Villa Park, with Amadou Onana already a long-term absentee and Johan Manzambi only recently returned to training after his own injury issues.
Goretzka's knee problem is not thought to be serious, but it means Emery must continue to lean on Ross Barkley, João Gomes, Lamare Bogarde and Boubacar Kamara to cover the position in the meantime.