Well so far we've swapped Donaldson for Barr, which should improve matters. If it were a one for one swap, I'd be worried but the club has stated it wants to go to a larger squad size which means that these guys, who are not nearly as bad as you suggest might ot be guarenteed starters.
I maintain the big issue we faced last year was squad size which meant a lack of competition, no cover for injuries and limited options for changing the style of play. Murray isn't inflexible about how his teams play - his first season should've demonstrated that, but he clearly caught himself out this year with a team that only had one real style of play. Further, we tended to lose stupid, stupid goals - not through playing utter garbage but through moments of breath taking stupidity, more often than not when we were on top. Individual errors can be sorted, systemic ones are harder to fix. A settled back four based around Barr and (hopefully) Watson won't leak nearly as many goals as our ever changing defence did this year.