Ertl is waiting for the results of a scan after sustaining what is feared to be a serious knee injury while playing for the reserves in midweek. Henderson and Williamson were sent off in last week's 3-0 defeat at Watford.
Striker Henderson, back in the starting line-up for the first time in 11 months after a hamstring injury against former club Watford last week, was sent off for two yellow card offences. Midfielder Williamson was shown a straight red card for his aerial challenge on Lloyd Doyley and starts a four-game ban as it was his second dismissal of the season.
Defender Matt Lowton has not trained this week and is doubtful due to a knock sustained in the reserves but midfielder Bjorn Helge Riise, on loan from Fulham, is back in the squad following a hamstring strain.
Source: PA
