Adam Clarke on Jeremiah 49:19

Behold, he shall come up like a lion - See the note on Jer 12:5. The similitude used here is well illustrated by Dr. Blayney: "When I shall occasion a like commotion in her (Idumea) as a fierce and strong lion may be supposed to do in the sheep-folds, then I will cause him (the man of whom it is said in the preceding verse that he should not dwell in it) to run away from her as the affrighted shepherds and their flocks run from the lion."

A chosen man - Nebuchadnezzar. That is, God has chosen this man, and given him a commission against Idumea.