The Blues must try to shake off their poor domestic form as they host FC Porto at Stamford Bridge knowing a draw will be enough but a win would see them top the group.
Porto have won 5 out of their last 14 away in Europe and drew the other 9.
Chelsea boss Jose Mourinho was asked if coming up against his old side makes a big difference to this encounter.
The Gunners must beat Olympiakos by two clear goals or score three if they're to squeeze through in second place in their group.
Arsenal have won seven and lost four of their last 13 away fixtures in this competition and are missing some big players including Alexis Sanchez and Santi Carzola.
They've lost on their last three visits to the Greek outfit – who sit in second spot in the group and are unbeaten at home in their last six games against English teams.
Manager Arsene Wenger claims that adds extra pressure for their opponents.
