The former Notts County star is viewed by manager Neil Warnock as a direct replacement for Jon-Paul McGovern who has returned to Celtic after a loan spell at Bramall Lane.
Notts County's Marcel Cas (left) is challenged by QPR's Paul Bruce during the Nationwide Division Two game at Meadow Lane, Nottingham.
"Marcel is a right winger with plenty of ability," Warnock told the club's official website.
"He gives us cover and we will have a good look at him until the end of the season."
Cas has flown out to Portugal with the rest of the Blades squad for a mid-season training camp.