The 25-year-old has suffered knee problems before, most notably throughout much of last season, but his current injury is unrelated - although Lancaster remains in the dark as to whether he will be available to face Australia at Twickenham on November 2.
"It's recommended that Alex has a couple of weeks' rehab. He's not out of the series by any stretch of the imagination," said Lancaster.
"Everyone is optimistically hoping he will come through and be available for some, if not all, the games.
"At the moment, having had fluid drained off the knee, he will have a period of rehab that covers this week and leading into next week. We'll see how we go, we're keeping an open mind on it.
"Long term it's obviously something he's struggling with, but we're optimistic that with good management - and his club Northampton have been excellent - we'll see him through beyond 2015.
"We're still quite a way away from the Australia game. We'll give him plenty of time but we'll do what's right for Alex."