Assistant boss Paul Goddard says: "He will be sorely missed, as he was when he missed games with a knee injury earlier in the month. He has had the groin problem for a while and the has been playing in quite a bit of pain.
"It came to a crunch on Wednesday and he was disappointed with his performance in the first half and said 'I'm sorry, I'm letting everyone down, and I'm struggling with my mobility.'
"It wasn't the Michael Carrick we know at Southampton and we need him back fit.
"We are looking at a long period of time and we don't want to put a date on his return but the main thing is he has got to be right and there is no point in us trying ot persevere as he is now - he has got to be right for us and England.
"Without a doubt his World Cup hopes are hit by it because when you get an injury you are out of the picture.
"I think there is enough time left in the season for him to show what he can do when he comes back from his injury - but in the mean time there are some massive games in front of us.
"We have got Lomy back, John Moncur, and Richard Garcia, who is progressing well and these are the boys who are going to have to knuckle down and do well for us.
"But there are no resources to add to the squad, so we will have to work hard with what we have got.
"We decided to bring Steve Lomas in and shuffle Joe to the left after Michael went off, but obviously he will need an operation, which means another reshuffle from now."
As for the defeat at St Mary's, Paul says: "It was disappointing for a number of reasons. We felt we were quite comfortable in the first half and were playing pretty steady ourselves, creating a number of half chances without Southampton causing us any problems.
"Then on the stroke of half-time again we gave away a sloppy free kick and then a sloppy goal that Southampton didn't even know they had scored.
"The game took on a different dimension, and within 10 minutes of the second half we found ourselves 2-0 down having not played too badly.
"Then we had a mountain to climb and the most disappointing thing was we couldn't muster anything in the last half hour to get us back in the game.
"We've talked about the game but you can't dwell on it too long because there are a number of games coming up.
"We have just had three tough away games and it has put us under a bit of pressure because it is very tight now."
As for tomorrow, and the need to avenge the 7-1 defeat earlier in the season, Paul says: "We were very disappointed and you don't want many days like that.
"No disrespect to Blackburn, but irrespective of that they are three points behind us and if we get a win tomorrow there will be a nice gap.
"It is getting to the part of the season where you start handing out medals now, and it is a vital time coming up - we are all well aware of that and we have to be motivated on our training and getting these results that we need."