Shaw, 29, is hitting just .162 with six home runs and 13 RBIs in 65 games this season. Because of Shaw's struggles, Mike Moustakas has been the Brewers' starting third baseman after starting the season at second base.
The Brewers had also optioned Shaw to Triple-A on June 28, only to bring him back a month later. He has hit well at the Triple-A level, with a .286 average, nine home runs and 22 RBIs in 28 games.
His season-long slump has come a season after he set a career high with 32 home runs. His average, however, dipped last season to .241 after he had hit .273 with 31 home runs and 101 RBIs in 2017.
The reigning National League MVP told reporters earlier this week that he felt something in his back during Monday's game, when he hit his major league-leading 38th and 39th home runs.
He said the current injury is similar to the issues that have sidelined him at various points throughout the season.
Friday will be his third straight missed game.