Nelsen picked up an apparent knee injury in the win.
“Having only looked at it on the replay I’m hoping that he’s twisted his ankle and not his knee,” Allardyce told the club’s official website.
“But he was holding his knee which was the big problem for me.
“I think it is his one where he’s had surgery before, so it’ll be 48 hours before we can tell and we’ll keep our fingers crossed, not just for us but for him.”
