Did you watch the game? The play Duvall misplayed was a sure third out and not called an error. A runner scored on the misplay and the next guy homered. It should have been 3 unearned runs. If you can't count that one as poor D leading to runs, I can never convince you of anything.
Again....
With average defense, I am convinced we'd have won 8-9 more games, minimum. With slightly above average: 9-11. Great D 11-12 or more.
Box scores show errors and unearned runs. We've already seen dozens of plays that used to be called errors caalled hits, this year, not to mention all the plays they've never called erros, like an OF'er circling a fly ball and never touching it.
Opposing teams had a .309 BAbip vs us, this year. That means about 31% of all balls put in play were hits, while 60% were outs.
Since 183 of the 1239 hits were for HRs, 1066 were balls hit for singles, doubles or triples. (That's not counting reach on errors, and we have had 94, some of which did not allow a baserunner to get on base, but did allow him to advance a base or two.)
The overly generous scorekeepers already shows our errors,alone led to 50 unearned runs, which in theory led to 5 losses. I'm 100% certain, uncalled errors that were clear misplays led to another 30 runs and 3 losses, and marginal plays not made led to 1-3 more losses.
Of course, no game is one or lost on one or even 2 plays, but losses could have been converted to wins with better pitching, better batting, better defense and better baserunning.