Elias broke his arm...

...but apparently broke it up to two weeks ago. Didn't occur to him to complain much about it until he used it to break the fall that led to a nasty gash in his forehead. Given how much he and Finn have been training to become ninjas and/or Jedis, his first reaction after seeing the x-ray makes sense:

"Well, Finn's going to have to fight on his own for a while."