Red Sox not done addressing their pitching staff this offseason

The Boston Red Sox wanted to "raise the ceiling" of their pitching staff. That mission has been partially accomplished.

The Red Sox began addressing the pitching staff with a significant bullpen upgrade. Aroldis Chapman signed a one-year deal worth $10.75M, adding a hard-throwing lefty for late in the game and a fallback option if presumed closer Liam Hendriks cannot return to form.

The rotation was next. The Red Sox made a blockbuster deal with the White Sox, adding starting pitcher Garrett Crochet for a quartet of prospects. Crochet gives the Red Sox a sorely needed top of the rotation arm, taking pressure off of young pitchers Brayan Bello, Tanner Houck and Kutter Crawford. 

Adding Crochet became even more important as the Red Sox look to keep up in the arms race with the Yankees, who agreed to terms with starting pitcher Max Fried on Tuesday.

Those moves do not mean that the Red Sox are done addressing the pitching staff. Jon Morosi from MLB Network reported that the Red Sox are attempting to acquire one of the Mariners' starting pitchers. Daniel Cramer from MLB.com reported that speculation is that Luis Castillo is being made available. 

However, one player on the Red Sox would not be heading to Seattle in a possible deal for Castillo; Alex Speier of the Boston Globe reported that first baseman Triston Casas would not be part of any potential trade.

A trade for Castillo would make sense for the Red Sox. The right-hander posted a respectable 3.64 ERA and a 1.169 WHiP in his 175.1 innings, striking out 175 batters with 47 walks. Castillo has been a workhorse, making at least 25 starts in the last six non-pandemic seasons. He could be the missing piece in the Red Sox rotation.

The Red Sox have improved their pitching staff by adding Chapman and Crochet. While it is a great start, those two moves cannot be all the Red Sox do on the pitching front.