Blackpool assistant manager Steve Thompson, who took charge of the side as Ian Holloway was away checking on a mystery transfer target, fears Southern could face a lengthy spell on the sidelines.
"As a player you know when something is not right and he thinks something has gone in his knee," Thompson told Sky Sports News.
"It is early doors and we have our fingers crossed that hopefully it is not as bad as we first fear.
"But Keith has had a few problems with his knee so I think he knows.
"And we have Gary Taylor-Fletcher who has stitches in his shin so we are two down today and we are down to the bare bones at the moment."