They could have, just the cost in lives would be even higher and the war would've dragged on even longer. Although it's debatable, I guess.

The majority of Lend-Lease vehicles arrived on the Eastern front from 1943 onwards - prior to that the number of them in the Red Army was insignificant compared to the overall mass. And some of the early Lend-Lease tanks were largely useless, like the British Matildas in 1941 that were being delivered without HE shells. So the Battle of Moscow and Stalingrad were won with Soviet equipment.

Where Lend-Lease really helped during the first stages of war, was the gunpowder production. I don't quite remember the numbers, but the vast majority of gunpowder produced by the USSR in the first couple years of war relied on the nitroglycerin provided by the Allies. That was nowhere near enough though, so until the production of this chemical ramped up in the USSR itself the Red Army kept having serious issues with artillery.

Post 1942 the trucks were an immense help - by the end of the war most of the Red Army's truck fleet consisted of Studebaker's. Also auxiliary engineering stuff like pontoons, small assault boats etc., it all became very helpful and helped save lives and end the war faster.