Barnsley strikers Obbi Oulare and Aaron Leya Iseka's debuts are being delayed by VISA issues, although the Belgian duo are also not yet fully fit.
The Chronicle understands that they are both training away from the Reds squad as they wait for the final paperwork from the Home Office.
Head coach Markus Schopp said: "They will not be available for the next game. I hope we will have them when the international break is over (in mid-September).”
Meanwhile, striker Carlton Morris is set to miss six to eight weeks after injuring his knee against Coventry City on Saturday.
Schopp "The information I have is that Carlton will be out for six to eight weeks.
"It’s a tough one for us. He is a guy who gives us a lot.
"We were fortunate because it could have even been worse.”