Williams was due to return to the Blues squad for this week's Heineken Cup clash with Edinburgh but a training ground incident now looks set to keep him sidelined until the New Year.
"Martyn was in the squad for Friday's game with Edinburgh but he had a bit of a knock," said Blues boss Justin Burnell.
"He was the first to come in on Monday morning to tell me he was available and could be announced in the initial squad…but he's had a bit of a setback unfortunately.
"He was taken from the field with our physios and we're just waiting on the results for the X-rays on his arm.
"He has been fantastic for the period when he was injured. We're hoping it is not too serious."
But while Williams, a Lions tourist in 2001, 2005 and 2009, faces more time out, fellow Welsh international Gavin Henson is on the verge of a comeback.
The high-profile centre joined the Blues in October but hasn't yet featured for his new region due to the wrist injury that scuppered his World Cup hopes.
Like Williams, Henson had initially targeted the European double header with Edinburgh as a possible return date but the 2005 Lion is at least expected to feature in the festive derby with the Dragons on December 23.
"Everyone is excited about Gav. We were informed by our physios that he will be taking full part in training on Thursday," added Burnell.
"Then let's get a full week of training next week then hopefully start pinpointing some games to come back.
"Obviously we've got he Dragons on the 23rd and the Ospreys on New Year's Day and hopefully he can take part in one of those games."