Nikkan Sports filed a report this morning that Yu Darvish cut his bullpen session short yesterday (he was supposed to throw around 50 but stopped after throwing 37) because he couldn't get comfortable on the mound. Seems it had something to do with his right elbow. And while the report also mentions that his start start tomorrow was canceled as a pre-caution, Darvish tweeted today that the elbow was feeling much better today. He also tweeted that he stop his session short yesterday as a means to avoid injuring himself.
So for those wondering, it seems he's ok.