According to Boston Globe article today, the 3rd cortisone shot seemed to do the trick. Its apparently an uncooperative nerve that is the source of the pain. Hayward is convinced it is a by-product of his break back in 2017.
https://www.bostonglobe.com/sports/celtics/2019/12/24/gordon-hayward-ready-return-action-celtics-christmas-game/4KOnhMF2m7JoxchVHfLzYI/story.html
Of course he is, he hasn't been able to stop catastrophizing the injury since it happened.
Paul George was literally back on the court the same year he had his injury. He played the last 6 games of that season, and although he wasn't 100% in those 6 games, his raw per 36 stats were 21 pts-8.7rebs-2.4ast-2stls in his first 6 games back (again the same year he was injured).
We're going on 3 years later for Hayward - he was a shell of himself in his 2nd year post injury and now we're a 3rd of the way through season 3 post-injury he's played 11 games.
Maybe Utah Hayward just never made it to Boston.