Proudlock, who has excelled under Paul Sturrock, has had a major setback after breaking his fibula in training.
It is expected that the striker will be out for at least six weeks but most likely he will be on the sidelines closer to two months.
Meanwhile, Doncaster-born midfielder McMahon has been instrumental in Wednesday's three-match winning run in the league but picked up an ankle problem in the 4-1 win at Bristol City.
With Chris Marsden out, it leaves only Glenn Whelan as the only first-team player who is at home in central midfield.
Sturrock could play winger Matt Hamshaw in the middle following his success in the middle of the park in pre season but he is usually used as a substitute wide man.
The Wednesday boss said: "Lewis is going to be out for a wee while so we will have to chop and change again. We'll have to start looking very seriously for a midfielder now."
One player who could be of interest of Sturrock is Tony Dinning, the Wigan star who completed a loan spell at Ashton Gate at the weekend was a target of The Owls before his switch to the South West.
There is still no news regarding Dennis Hobson's cash injection to fund a loan player although the board will meet 'within 24 hours' following the money being put into a holding account.