O'Toole, 20 has joined on loan from Championship rivals Watford until the remainder of the season and will link up at Bramall Lane with former Hornets Leigh Bromby and Darius Henderson.
United will welcome back on loan players; Leigh Bromby, Artunro Lupoli, Jamie Ward and Craig Beattie who were all cup-tied for the fifth round replay at Hull City last Thursday night.
However Blades boss Kevin Blackwell will be without skipper Chris Morgan who picked up his fifth booked of the season in last weekends 2-2 draw at Plymouth Argyle and under the seven day rule will be suspended for the visit of the Blues; whilst striker Darius Henderson returns from suspension and is likely to make his first Championship since his straight red card he received at Southampton.
Although likely to feature on the substitutes bench the visitors will be familiar with the face of Ian Bennett who served for the St Andrews club between December 1993 to June 2005 - making 352 (+2) appearances for the Midlands club.
Possible Blades Line-Up: Paddy Kenny, Kyle Naughton, Leigh Bromby, Greg Halford, Gary Naysmith, David Cotterill, John-Joe O'Toole, Brian Howard, Stephen Quinn, Arturo Lupoli, Darius Henderson.
Birmingham City will be looking to get back on track after drawing 0-0 at ten-man Crystal Palace in midweek, but that point put them in good stead as they occupy an automatic spot before the weekend.
Blues boss Alex McLeish will travel to Bramall Lane without duo Sebastian Larsson and Lee Bowyer; the pair have picked up five bookings for the season, and like Chris Morgan will be suspended for this fixture.
He will also be without former Liverpool man Djimi Traore (hamstring) but Gary McSheffery, whom has been linked with a move away from St. Andrews may feature for the first time since Christmas.
Possible Blues Line-Up: Maik Taylor, David Murphy, Stephen Carr, Rahdi Jaidi, Liam Ridgewell, Keith Fahey, Lee Carsley, Kemy Agustein, Scott Sinclair, Marcus Bent, Carlos Costly.