Club doctor Ges Steinbergs reveals: "Michael has been struggling for some weeks with his lower abdomen and he has gone in for a hernia operation on both sides."
Michael came off at half-time in the defeat at Southampton last week, and Ges explains: "At Southampton it was just so painful he couldn't carry on. The decision was the right thing because, although he has been battling on, it sometimes it takes a toll on your performances.
"But he has come through very well and we are very confident the surgery will be okay. Now it will be a matter of five to six weeks before we are getting him playing again, provided there are no complications along the way.
"We are very happy with our surgeon, Steven Snookes, who provides our groin operations - and we have always been very successful with them.
"Certainly six weeks would be our target, though sometimes we have managed to get people back a week or so earlier.
"The important thing is it needed doing. We need as many players fit as possible to help our cause, but I don't think Michael could have struggled through."
The hope is that Michael's World Cup finals ambitions won't be affected by the injury, and Ges adds: "I hope, if it goes according to plan, he will have a couple of months to prove himself fit and put in some good performances.
"Perhaps, with his groin being back to normal, his performances may improve and he can get back into Sven-Goran Eriksson's England squad.
"I would hope he could make it on merit because he has had a good two seasons."
Weekend reports that Ian Pearce returned to training on Monday are inaccurate. He is already on the comeback trail, and Ges says: "It is good to see him, and it is much more positive. We now feel we are moving forward after a long period of injury and illness, and he is putting in the effort in training.
"There is little reaction, so we are very hopeful for him."
Fredi Kanoute will definitely be fit for Chelsea on Wednesday, barring unforeseen mishaps, despite requiring stitches at the weekend.
"Fredi took a blow from an elbow just before half-time, and he got a small cut," says Ges.
"We just let him get through until half-time and we put three stitches in, but there is no real bruising or damage to the eye, so he should be absolutely fine for Wednesday."
Ges admits, though, that Trevor Sinclair is not an absolute certainty after returning to action after a two match lay-off, and says: "Trevor has had a very painful shin after a total of nine stitches put in. He was still slightly sore on Saturday but he is a solid performer who likes to get in there even though he is in a bit of discomfort.
"He turned in a fantastic performance on Saturday even though he was still sore, and there is a bit of work to do before Chelsea to keep him playing but, knowing Trevor, I'm sure he will want to be on the pitch for that game."
Youngster Izzie Iriekpen is hoping to continue his trail back to full fitness after a three year interruption to his fledgling career, and Ges adds: "Izzie looked impressive in a friendly at Stevenage last week, and the most important thing was to get some time under his belt and for his knee not to react.
"The early signs are good, but he will have to have a few more games before we see how things go for him."