Cardiff Blues will be without Jamie Roberts for their 14 January Heineken Cup clash at London Irish.
The Wales centre limped out of Cardiff Blues' 28-9 Pro12 win over Newport Gwent Dragons on 23 December.
It is still expected Roberts will be fit in time for their Six Nations opener against Ireland in Dublin on 5 February.
The Blues had originally hoped he would be fit for the crucial London Irish clash.
Roberts twice had treatment after coming on at the break against the Dragons and left the fray after 64 minutes.
While Roberts' knee was being treated on the field before his substitution, Dragons full-back Martyn Thomas collided with him and the medical staff.