Honestly, I fear more about Luis more than Cahill. At their best, I think Luis is the better defender, I really do, but he tends to have matches where his head seems to be elsewhere and he ends up making more mistakes than good decisions. He had a perfect game against Tottenham, but was the worst player on the field against Shakhtar imo. If Luis has a good game, I am confident we will do fairly good at the back.