Murphy suffered a groin injury during Tuesday's 3-2 win over Preston North End.
Ian Baraclough, the Iron manager, has ruled him out for Saturday's trip to Watford and brought in Warner, who will start ahead of Sam Slocombe at Vicarage Road.
"I felt as if we needed a bit more experience and, with Tony Warner being a free agent, available we felt it would be a good move," said Baraclough.
"He's been training with Liverpool and played for Leeds United against our reserves last week. It seemed a no-brainer.
"He's a good talker, an excellent shot-stopper. He's been around a good few years and his experience will enhance what we've got in the back four."
Warner joined Leeds two weeks ago to provide cover following injuries, and played for their reserves in the 1-0 loss to Scunthorpe's second string.
He has signed a one-month deal at Glanford Park. He has played for 13 different clubs, with his only real spell as first-choice keeper at Millwall from 1999-2004.