Antony found a place and opportunity at Betis. His performance is indeed different than when he was at United. It could be due to the different team situation or the different competition.
Hopefully, his performance can improve this season. Because last season he performed quite well with Betis, with 17 matches, 5 goals, and 2 assists. A result he could not achieve when he was at United.
You have to understand that players have careers too; they can't just sit on the bench; their value plummets. So if they're ambitious and think they can make it, their goal is to play, amaze, and lead their team to victory so that their value rises. That's how they build careers.