Barnsley defender Aidan White and winger Saidy Janko face a lengthy spell out after picking up injuries.
Janko, 20, and White, 24, were missing from the line-up for Saturday's 2-0 defeat at third-placed Brighton.
"Saidy's torn his thigh and will be out for a few weeks," head coach Paul Heckingbottom told the club website.
"Aidy has a little tear and has to rest until he feels comfortable with it. It's not a major muscle so you can't put a timeframe on it."
Winger Janko has scored one goal from two substitute appearances since joining on loan from Celtic in August.
"It's disappointing because he's come in and made an impact from the bench," said Heckingbottom.
"We were looking at getting him to start games as we came into this busy period, but he's out of the picture now until after the international break."