Lend Lease only accounted for 14% of total war time production for USSR. The real lend lease came in the latter half of 1943, when major battles (Kursk/Stalingrad) had be won and the Wehrmacht was on retreat. Lend lease though helpful, was not critical at all.You need to read up on second world war and how USSR fought. You need to learn about how USA helped USSR though lend and lease.
EDIT: Not all lend lease reached USSR (which was not the original intended recipient for it), a lot of shipment was lost at sea.