There is a little bit of damage in that but not as bad as it might have been, still nothing below four to six weeksPep Clotet.
Roberts sustained the injury midway through Blues' 1-1 draw against Millwall last weekend after he landed awkwardly during an aerial challenge.
“It is not as bad as it could have been,” said Blues' Head Coach Clotet.
“Obviously when we think that Marc Roberts is a very big player with big weight.
"All the weight went into one ankle and it could have been damaging, he could have broken the ligaments.
“There is a little bit of damage in that but not as bad as it might have been, still nothing below four to six weeks.”
Roberts' absence should ensure that Chelsea loanee Jake Clarke-Salter will continue his run of games, but alongside captain Harlee Dean who was available from last weekend after serving out a three-game suspension.
Meanwhile, Clotet says he expects Gary Gardner (knee) to be available for selection once he has gained some match practise for the Under-23s in the next week or so.
Maikel Kieftenbeld (knee) is still pencilled in for a return some time in the new year.