Schwarzer broke a thumb in training on Thursday afternoon and Boro boss Bryan Robson was given special dispensation to bring in an emergency signing from another Premiership team.
The rules state that a Premiership club loaning a player from another top-flight side is forbidden but Boro have been allowed to do so because of their goalkeeping crisis.
Marlon Beresford is still recovering from a cartilage operation, summer recruit Mark Crossley is still out with a hamstring tear and teenager Bradley Jones is unavailable with glandular fever .
"It's incredible," said Robson. "I think when I was at Manchester United a goalkeeper was injured once in 13 years, yet ever since I have been at this club it has been a problem."
"With the other lads injured the last thing I thought would happen would be an injury to Mark Schwarzer."
It is believed Robson had made an earlier move to bring Schwarzer's Australian compatriot Mark Bosnich to The Riverside Stadium from champions Manchester United, but Bosnich turned down the offer of first-team action.
Walsh, who cost Boro £500,000 when he joined from The Red Devils in August 1995, has lost his place in The Bantams' first-team to the in-form Matt Clarke and is likely to make his 59th appearance for Boro at Manchester City on Sunday.
To read Keeper Crisis for Boro click here.
