The 23-year-old was forced to undergo surgery on his fractured skull.
“Steven has been so unlucky with injuries at Derby County and we’re all gutted for him,” Derby boss Nigel Clough told the club’s official website.
“If anyone is due a bit of luck, it’s him.
“It was an innocuous incident as well with no intent from the Southampton player at all.
“He is going to be out for the best part of 12 weeks and we will miss him.”
