This is not credible because you have zero hard evidence. Go to all the box scores--or better still the pitch by pitch replay you can get on mlb.com--and cite specific examples of 8-12 games lost specifically because of the defense and nothing else.
I hasten to add that the Sox lose in 9 innings last night without the two Rays errors which led to 2 unearned runs. The two Sox errors led to 0 unearned runs.
The Sox are ranked 21st in team ERA. I know you and many others despise ERA, but over time it's been a consistent marker on whether or not the Sox will get to the postseason. Below are the Sox rankings in team ERA and whether or not they made it to the postseason.
2022, 25th, no
2021, 15th, yes
2019, 19th, no
2018, 8th, yes
2017, 4th, yes
2016, 9th, yes
2015, 25th, no
2014, 23d, no
2013, 14th, yes
2012, 27th, no
2011, 22d, no
2010, 22d, no
2009, 16th, yes
2008, 9th, yes
2007, 2d, yes
2006, 26th, no
2005, 24th, yes
2004, 11th, yes
2003, 17th, yes
2002, 7th, no
2005 (ERA 24th, yes postseason), and 2002 (ERA 7th, no postseason) are the two exceptions in 20 seasons. I skipped 2020 because I don't think it was a real season. But, FYI, the Sox ERA was 28th and they didn't make it to the postseason.