Japan will do well initially simply because the Soviet union is preoccupied with the Germans. This will have a very detrimental affect on Lend Lease sent to Vladivostok; that will need to be diverted to the long route through Persia. Long term success for the Japanese is more dubious, but in conjunction with the Germans a favorable negotiated peace may be possible.
The strategic problem for Japan in this scenario is China.
No Sino-Japanese War and an invasion of the Soviet Union in 1941 means Nationalist China has four years to consolidate its control of the country, build its economy, and improve its armed forces.
Chiang will have eliminated the Communists by then (at least an independent political force with its own military), and likely imposed his control over one or more of the remaining warlords (probably Guangxi and Han Fuqu of Shandong).
Economically, China will have completed the various Three Year projects begun in 1936. It will have new arsenals producing rifles and artillery, and its first factories producing automotives, radios, and essential chemicals for armaments. It will also be well on its way to completing a second group of Three Year projects.
In the military, it will have a Central Army of around 60 trained German divisions (as opposed to the seven it had at the start of the Sino-Japanese War). It will probably also have a reformed air force (Claire Chennault was actually hired by Chiang before the Sino-Japanese War began).
So it is a far more formidable country with these extra four years.
If Japan goes to war with the Soviet Union, it presents Chiang an excellent opportunity to move against Japanese interests in China. Most likely this means establishing control by Nanking over Hebei and Shanxi. That means ending the "East Hebei Autonomous Council" which was a Japanese puppet organization. Preoccupied with the Soviets, the Japanese will have a hard time doing anything than verbal protests.
Once that is satisfied, the Chinese will look at how they can get back Manchuria. Most likely Chiang will wait for signs of difficulty for Japan. Depending on the exact circumstances, China could pursue a negotiated or military solution.
The Soviets can potentially concede enough to Chiang to make it very attractive for China to enter a war against Japan. Stalin can abandon Xinjiang and even Mongolia to China, and give up any claims in Manchuria.
Despite the qualitative edge of the Japanese troops and Japanese economy, by the mid-1940s, Nationalist China will have improved enough qualitatively that their superior numbers can win a war against Japan. Their formations will not be equivalent, but they'll be good enough given their numbers.