The US learned important lessons from WW II but institutional amnesia wiped out crucial information about the functioning of small units. The debacle of replacing one man at a time in VietNam was the tragic result.
The World War II experience with the practice of dribbling "replacements" into tired and depleted units wasn't much better.
The "concept" was to keep divisions in the line continually, and not to pull them out for R&R (which is what von Bittrich's II Panzer Corps was doing when the British dropped on them at Arnhem and found them rested, resupplied, and re-equipped).
The American practice ground up the replacements very quickly -- part of it was lack of esprit de corps, which denied the replacements the quick OJT and morale improvement that comrades would/should have afforded them otherwise. Instead they became anonymous zombies, shunned as "unlucky" and cannon-fodder by the veterans (which became a self-fulfilling prophecy).
I saw a statistic once -- I think it was in Patton's latest bio -- that only 10% of the troops present in the Normandy divisions when they crossed the Rhine had landed in the invasion. The rest had gone home in hospital ships or pine boxes, or been buried in war graves.