The tough-tackling 26-year-old was carried off with suspected medial ligament damage during the Tangerines' pre-season friendly loss at Bristol City on Saturday.
Southern will still miss the start of the Barclays Premier League season but his lay-off is likely to be weeks rather than months.
The former Everton trainee told the club's website: "I had a scan and I was fearing the worst, not only my medial ligament but my cruciate ligament as well.
"I had a bit of good news on Monday night and it's neither. I've torn a muscle that attaches to the kneecap, so I'm still going to be out a little while, but it's very positive in that I haven't damaged any of the main structures to the knee."