The Arsenal midfielder scored the opening goal during Germany's friendly defeat to Austria on Saturday, but was forced off 14 minutes from time after picking up a knee injury during a challenge with Watford defender Sebastian Prodl.
German publication Bild reports that Ozil has not returned to full training since the incident, instead being limited to fitness and conditioning work.
The 29-year-old missed the final seven Premier League games of the season due to various injuries, and is now not expected to feature in Germany's final warm-up game against Saudi Arabia on Friday.
The defending champions get their World Cup campaign underway against Mexico in 10 days' time, leaving Ozil as a doubt for the opening stages of the tournament.
Ozil has won 90 caps for Germany and was part of the team which lifted the trophy in Brazil four years ago.