He has improved. His fastball is up about 1mph from last year.
His problem is location. When Thornburg was an effective reliever in Milwaukee, he lived on the inner and outer thirds of the plate. Right now, he keeps missing inside and outside, which means his release point is off and the reason his walks have been so high. He’s been compensating by throwing the ball over the middle of the plate (like every pitcher trying to find the corners does), which is why he is getting hit harder.
Releasing Thornburg and keeping less talented pitchers in the pen like Marcus Walden or Brian Johnson for example, will be a huge mistake in the long run. The Sox need to let Thornburg rediscover avdelivery that finds the corners.
And they clearly know this.
At some point, they might decide this isn’t going to happen soon enough. But not likely in early May...