The Dutchman is currently back home in the Netherlands receiving treatment, causing him to miss the club’s official squad picture photoshoot yesterday (Thursday).
However winger-turned-striker Jordan Roberts and central defender Mark Connolly will be fit for selection for tomorrow’s home game against Notts County.
Roberts scored three goals in two games during wins against Swindon Town and Yeovil Town before suffering an injury meant he missed Reds’ last two matches against Colchester United and Stevenage.
Kewell said: “Unfortunately for Thomas the injury he suffered against Charlton at first we didn’t think it was too much, but it’s worse than we feared.
“He (Thomas Verheydt) is back in Holland, getting treatment.
“But Thomas is very positive, he is getting back, getting fit, we are in constant touch; he’s fully focussed on returning.
“He’s devastated, but that’s football. We all know we have to put our bodies on the line and we all know at any moment things can happen.
“Thomas is a fantastic player, has a good attitude and is a very hard worker. So they tick all the boxes for me.
“He will be out for up to a month - it’s a blow not just for himself but for the club.
“We are doing everything we can do to get him back as soon as, fit and ready to continue because we are going to need every single player.”
Kewell revealed: “Jordan did take an injury but he’s worked very hard with the medical staff here to get back so he’s looking good.”