The 26-year-old German suffered a hamstring problem during Saturday’s 1-1 Premier League draw with West Brom which saw him stretchered off the pitch.
And scans have since revealed the extent of the damage to the Black Cats’ key man.
A club statement said: “Jan Kirchhoff has been ruled out for a minimum of six weeks after suffering a hamstring injury during Sunderland’s draw against West Bromwich Albion.
“The former Bayern Munich player was stretchered off in Saturday’s game at the Stadium of Light and in order to assess the extent of the injury, the 26-year-old underwent a scan which revealed he will be out for six to eight weeks.
“The midfielder will now focus on returning to action as soon as possible.”
Sunderland, without a win in their first seven Premier League games, currently sit at the foot of the table with just two points.
Moyes, who replaced Sam Allardyce during the summer when he headed off for his ill-fated spell as England manager, has had to contend with a series of injuries in recent weeks.
Goalkeeper Vito Mannone, midfielder Sebastian Larsson and striker Fabio Borini are other long-term absentees from a squad which is looking thinner by the week.
Kirchhoff has proved a big hit on Wearside following his arrival from Bayern and his prolonged absence will rob Moyes of one of his more dependable players.