The Boston Red Sox designated left-hander Rich Hill — at 44 the oldest player in the major leagues — for assignment on Friday and promoted one of their top pitching prospects in right-hander Luis Guerrero.

Boston had signed Hill as a free agent to a minor league contract on Aug. 15 and he later joined his hometown team for a fourth stint. Hill became the only active player to appear in a major league game in each of the last 20 seasons when he pitched in relief in an Aug. 29 loss to the Toronto Blue Jays.

Hill was 0-1 with a 4.91 ERA, three walks and five strikeouts in 3 2/3 innings across four relief appearances.

His career record is 90-74 with a 4.01 ERA, 543 walks and 1,428 strikeouts in 1,409 innings over 386 games (248 starts). Hill pitched for the Chicago Cubs (2005-08), Baltimore (2009), Red Sox (2010-12, 2015, 2022, 2024), Cleveland (2013), Los Angeles Angels (2014), New York Yankees (2014), Oakland (2016), Los Angeles Dodgers (2016-19), Minnesota (2020), Tampa Bay (2021), New York Mets (2021), Pittsburgh (2023) and San Diego (2023).

Hill’s designation opened a roster spot for Guerrero, 24, ranked the No. 28 prospect in the Boston system by MLB Pipeline.

Guerrero has a 3.31 ERA with 79 strikeouts in 42 games (an average of 13.09 per nine innings) with Triple-A Worcester this season. He has allowed one run in his last 10 outings dating back to Aug. 4 for a 0.68 ERA with 23 strikeouts in 13 1/3 innings.

The native of the Dominican Republic moved to Boston as a child and grew up in the area. The Red Sox selected him in the 17th round of the 2021 MLB Draft.