Warriors rugby director Mike Ruddock said: "Pat has had a setback with his injury.
"He has ended up having to have some blood drained out of his leg and go to hospital.
"There is quite a lot of blood under the muscle that has congealed and needed to be drained off. He has also had a bit of a reaction to some painkillers.
"But he is a tough guy, and if anyone is going to make a rapid recovery after something like that, it's going to be Pat Sanderson.
"It is probably going to be a number of weeks before he returns to action."
Worcester head to Premiership champions Wasps on Sunday, before hosting Leicester next week then visiting Bath on September 27.
Former New Zealand lock Greg Rawlinson, who led the side against Northampton, will continue as captain in Sanderson's absence.