The former Wolverhampton Wanderers Midfielder had the chance to join the Greek side on loan and was due to make a decision within the next few days but decided a move to Bramall Lane was a better move.
Neil Warnock told sufc.co.uk: "Carl is a player who has made it known he wants to play for us and he's the type of player we need.
"We lack numbers in that area of the field and I feel this is the right time to see what he has to offer.
"He's mates with Robert Page and hopes a move here will help him get back in the Welsh squad."