Why would this be the case? It seems that Bukharin would make the USSR stronger and more reasonable.
The popular thread of reason on this forum is that "more free market = stronger USSR", but as far as I understand it the academic consensus (from the more recent social histories) is that the NEP was alienating the primary constituency of the Bolsheviks (the urban workers) and was creating contradictions within the Soviet economy that were heightening to a pitch (think the issue with the NEPmen). Bukharin's advocacy for a continuation of the program would have imposed his own vision on the NEP that probably would've contributed to a disintegration in the fledgling Soviet administrative apparatus and weakened its ability to combat the Germans when they came roaring over the border in '41. I'm not particularly well read on this in particular, so I would have to do some more reading for a more in depth answer but that's what Carr was essentially claiming..