by eastern theatre do you mean "european near east" (syria, egypt) or "soviet far east" (such as in "japanese landing in kamtchatka")?
I think he meant an American presence in Russia itself. And I think it's highly unlikely; Russia doesn't need more men. What it needed most out of the US was equipment and food supplies, so they could support and equip the men they had.
That said, maybe a Communist revolution in the US in the mid-1930s, so the US is much more friendly to the USSR and shunned by the rest of the Western world? That makes the Brits and the French much more willing to compromise with Hitler, and stay neutral when Hitler attacks Poland. Stalin, however, doesn't want a fanatically anti-Communist madman on his border, and intervenes in the Polish war "to save the oppressed Poles from the bourgeois tyrants" or some such propaganda. He bites off more than he can chew, however; Hitler's armies push the Soviets completely out of Poland, and well into Russia itself. The weaknesses uncovered by the Winter War OTL are being uncovered now, against a much more dangerous enemy. The Soviets are pushed farther and farther back into Russia, and take large losses. The Brits and French are getting kinda nervous by now; they welcomed the German-Soviet War initially, but now that Hitler is making such progress, they wonder what Hitler will demand postwar. However, they don't give the Soviets aid yet, instead retreating into stony neutrality.
The US, on the other hand, gives the Soviets large amounts of aid from the start. As the Soviets keep getting pushed back, the US realizes that it isn't doing much good, and start to think about much more drastic aid. After Kiev falls, the US declares war on the Germans. Since a US lading in Germany itself is completely out of the question, the US sends whatever troops it can raise to help the Soviets, as well as larger amounts of material aid.
Since the Europeans aren't weakened by the war in Europe, the Japanese are dissuaded from attacking the European colonial empires in East Asia. In fact, the Brits and French start selling the Japanese strategic materials after the US stops selling them to Japan after the Japanese attack on China. They see Japan as a useful counter to Stalin in the Far East. So the Japanese, seeing an opportunity now that Stalin is distracted with Hitler, declare war on the US and launches a surprise attack on the US.
How's that for a (very very) rough scenario? I admit, it has problems, but it may be a good template.