It wasn’t the fall of Greece, but the Pro-Allied coup in Yugoslavia, in March 1941, that set the timetable for the invasion of Russia back.
There is considerable debate among historians as to whether or not the Balkan invasion impacted the Russian timetable. The original date for Barbarossa was May 15, but the Germans delayed so they could further discuss strategy and logistics for the coming invasion.
I am sure the Balkan invasion affected the availability of tanks and transport aircraft that could have been available for the Russian campaign. A lot of JU-52s were lost over Crete (AND CRETE SHOULD NEVER HAVE FALLEN--but that's another story!) Let's not forget the other factors that impacted the Germans as they marched across Russia:
As a Greek-American I am proud of Greece's resistance to the Italians and Germans. It made Mussolini look like the idiot he was. Both of my grandfathers served in the Greek army and later in the resistance. They both had to hide from the Germans as they came looking for all the males in their villages to be taken away and shot. I will never forget the story as my grandfather was hiding, he could hear the machine gunning of the men in his village. The Germans later dumped the bodies into a well. Today that well is sealed with a memorial on top that lists all the names of the executed men. I remember him crying as he told me the story. He lost so many friends.
The thought of that makes me shudder for if they were both captured and shot, I would not be here today.
Memory eternal to the heroes of 1940.
Germany had to support Italy when it was defeated by Greece. Valuable time lost for the Nazis.
Then on April 6, 1941, German troops invaded Greece through Bulgaria in order to establish a bulkhead on its southern flank. Again the Greeks withstood the Wehrmacht...again delaying and frustrating Hitler's plans.
There's more. Later in Crete with operation Mercury the Greek resistance dealt incredible casualties on the marauding Germans never letting up on the invaders. It was the first time the Germans met with incredible civilian resistance.
see this: