It isn't the clearest clip but it shows the Boro player arriving late, he never looks towards the ball, he barely jumps to challenge and after he's cleaned Ballard out you see his arm going back down. IMO he's gone to do him there.
It's at the very least a booking, he doesn't challenge properly he just wipes Ballard out. Ballard's leg actually goes above their players head he's caught him so hard. Others with a good view at the game say it's an elbow. Ballard's reaction say it's an elbow.
The ref bottles it and books the pair of them.