Wasn't there some comment from Nako on the sideline that said Arena wasn't sure Zahibo could go a full 90 because of a knock, but given the way the first half went, they were going to play him for 45. If no slight knock for Zahibo, sounds like Caldwell was just the injury cover - not because he had a good week of training or earned a start.
Caldwell is the kind of player that's easy to keep in MLS. Good attitude, low-cost, popular with fans/local guy, some versatility, ... I think he'll be here unless he is unhappy with his reduced playing time. As for Diego, I'd guess Arena still has ambitions for him. For this year, he seems to be relegated to our 'super-sub' - and, in a couple of appearances, he fulfilled that role. I still have to think it's all about confidence and that Arena is just the right coach for that.