Constant line drives to the rim and a lack of rebounding.
I agree that not having Nurkic on the court made Boston more likely to drive. I don't think that Nurkic being on the court would have actually offered us any more rim protection (as 42% 3p% shooting Horford would have pulled him out of the paint), but I think Nurkic's presence would have made the Boston players hesitate before driving.
I think the rest of our players should have been able to effectively rebound without Nurkic out there, so I reckon that's more on our players than on Stotts.
If it was me, I would have played Nurkic at the end, and trusted him to both cover Horford on the three point line, and come off of him to cover the rim when the time was right. I also would have trusted the rest of our players to rotate properly if the ball was kicked out to Horford.
Additionally there's a case to be made that our offence would have been more effective at the end with Nurkic setting screens, and rolling hard to the rim.
All that said, I don't think it was as clear cut a decision as some people are making it out to be. I reckon it was a line ball call, and by all evidence, probably one that Stotts got wrong. But I don't think it was an egregious error, that warrants people abusing Stotts over.