The Red Sox are paying for not acquiring a healthy shortstop this offseason
The Red Sox let Xander Bogaerts walk to San Diego on a monster contract. Losing him definitely hurt, but Bogaerts hasn't exactly lit the world on fire in San Diego since April. He has a .732 OPS and a 106 OPS+ on the season. Letting him go isn't the problem. Doing nothing to replace him is.
The Red Sox gave Kiké Hernandez the reigns as the everyday shortstop to begin the year, and their plan to do so backfired. His 14 errors at shortstop are tied for the league lead despite him playing the 24th-most innings at the position. To go along with the defense, he has a 61 WRC+ and has been worth -1.3 fWAR. Not great.
Hernandez isn't the only shortstop on the team that has struggled. Pablo Reyes has had a decent offensive season relative to expectations, but despite hitting over .300 he has a WRC+ of 90 thanks to not walking nearly enough and hitting for no power. Yu Chang, their best defensive option most likely, hasn't hit.
It's not their fault completely that Adalberto Mondesi hasn't played yet, but when you hedge your bets on an injured player, you can't blame anyone but yourself when he has a setback. Even when healthy, it's not like Mondesi would do much to help offensively anyway even if he can help defensively and on the base paths.
Red Sox shortstops have a 61 WRC+ (27th in MLB) and have been worth -1.2 fWAR (29th in MLB). They didn't even have to sign Xander Bogaerts or any of the top shortstop options, just signing anyone who can play a decent shortstop would help. Trevor Story returning should help, but they didn't have to navigate the first half with pathetic shortstop play.