Nikita Khrushchev, in his own memoir, stating clearly that the USSR could not have won the war on its own:

I would like to express my candid opinion about Stalin's views on whether the Red Army and the Soviet Union could have coped with Nazi Germany and survived the war without aid from the United States and Britain. First, I would like to tell about some remarks Stalin made and repeated several times when we were "discussing freely" among ourselves. He stated bluntly that if the United States had not helped us, we would not have won the war. If we had had to fight Nazi Germany one on one, we could not have stood up against Germany's pressure, and we would have lost the war. No one ever discussed this subject officially, and I don't think Stalin left any written evidence of his opinion, but I will state here that several times in conversations with me he noted that these were the actual circumstances. He never made a special point of holding a conversation on the subject, but when we were engaged in some kind of relaxed conversation, going over international questions of the past and present, and when we would return to the subject of the path we had traveled during the war, that is what he said. When I listened to his remarks, I was fully in agreement with him, and today I am even more so.

-Khrushchev, Nikita Sergeevich; Khrushchev, Serge (2004). Memoirs of Nikita Khrushchev: Commissar, 1918–1945. Penn State Press. pp. 638–639.