Waratahs playmaker Berrick Barnes suffered a dislocated thumb in the Sydneysiders' 27-26 defeat to the Cheetahs on Friday.
Barnes, who was making his first appearance of the season after missing the Waratahs' previous matches with a knee injury, appeared to sustain his latest knock after making a tackle on Cheetahs lock Lodewyk de Jager in the 17th minute.
Waratahs boss Michael Cheika said it's still too early to say how serious Barnes' injury is.
"It went back in, obviously, so we'll just see," Cheika said.
"We won't know for at least 48 hours. We've got to wait."
The loss to the Cheetahs was the Waratahs' third one in four games and their 11th defeat from their past 12 matches.
"We are obviously going to be heavily criticised and, first and most importantly, we've put ourselves in a difficult position on the table," said Cheika.
"We're a team who is carrying a lot of anxiety as it is from past seasons.
"So we're going to get a lot more of that now and we have to have the resolve to understand that we're going to be in a massive war - with ourselves predominantly - and we have to have the courage and the application to make the sacrifice to get out of it because no one else is going to think we can."