I assume you are talking about the U.S. practice.

The U.S. utilized an individual replacement system during World War II. Unlike in the German Heer, U.S. Army units were kept in the line essentially continuously; the 35th Infantry Division fought for 162 consecutive days before being disengaged, and that was only to absorb replacements and make the turn north to begin Patton's attack against the southern flank of the German bulge in the Ardennes! When U.S. units took casualties, a theoretically almost-unlimited amount of men were available to be sent forward on an individual basis to replace them. In the Heer, units accounting for men absent or in hospitals often had to wait for replacements arriving from their assigned wehrkreis (military district) in Germany and were forced far below strength as a result; the U.S. system handled this in a far more efficient, if seemingly impersonal, manner as seen below. Particularly beaten units were shifted away from the front lines entirely for disbandment or rebuilding, to be replaced by a "fresh" unit (the preponderance of unit types and organization changes in the Heer in comparison to the U.S. Army is another animal entirely).

In the European Theater of Operations, Section II of Circular 69, issued by Headquarters, European Theater of Operations, U.S. Army on June 13, 1944, governed the accounting of men on unit rolls. If a man was injured, wounded, or sick, but was expected to be back on duty with his unit within 24 hours, he would be noted as such and no changes to the unit strength would be made. If the period of absence was expected to be over 24 hours but less than five days, his status with his unit would be "attached unassigned." In company morning reports, these men were carried on unit rolls but listed as "absent." If a man was injured, wounded, or sick to the point where his hospitalization would be more than five days, he would be dropped from the unit rolls entirely ("dy to [ ] to lost to hosp per Sec II Cir 69 Hq ETOUSA 13 Jun 44") and assigned to the hospital’s “detachment of patients” until he recovered. Men who were absent without leave were to be carried on unit rolls for a period of one year, and men who were absent as injured, wounded, or sick, but not assigned to the hospital, were carried as such for up to sixty days.

When a man assigned to a hospital recovered within the period of the theater's evacuation policy (usually 90-120 days) he would be filtered back through the replacement system as a "casual" (at any point, 40 percent of men passing through the replacement system were casuals) and sent to a forward replacement depot for a maximum period of ten days. He would be returned to his former unit "whenever practicable;" if no requisition for a man of his specialty from his former unit came in during that period, he would then be considered a "free" replacement and assigned to any unit that had a vacancy in his specialty. This was to prevent unnecessary over-strength in some units, and unnecessary vacancies or under-strength in others. Many hospitals openly flaunted the theater policy, returning men as close as possible to to their old units whenever possible, or allowing them to leave of their own accord. In August 1944, the Twelfth Army Group authorized casuals to be sent back to the replacement depots serving the field army from which they had departed, and gave preference for unit return to those men which had served with them the longest time. In January 1945, both the Twelfth and Sixth Army Groups authorized the automatic return of all casuals to their old units; the amount of "above [ ] enlisted men fr [ ] dr fr asgmt to ret to dy this organization per Par [ ] [ ] DSO [ ]" in company morning reports increases dramatically after January 1945.

Using the individual replacement system, U.S. Army units would not have to wait for a man to get out of the hospital or a new one to be sent from the United States; a replacement could be in the line almost immediately regardless of the situation back home. A downside of the geographic position, size, and logistical situation of the United States during the war was that a nationwide "individual" system, in comparison to regional recruitment like in the United Kingdom or Canada, or a military district-based "individual-unit rotation" system like in Germany, was almost necessary. A proposal was made by the commander of the Army Ground Forces, General Joseph Stilwell, in early 1945 to earmark specific training units, unit classes, or groups of men for assignment to specific overseas units before shipping, as well as a proposal to ship individual replacements in "squads" (three groups of four men), "platoons" (four squads), and "companies" (four platoons) to increase morale. The former part was rejected as unworkable because it would mean predicting casualties for specific units six to twelve months in advance (the time it usually took from when a soldier destined as a replacement was inducted to when he met his unit on the front lines), and the latter part went into effect so late it was hard to measure if it had any impact.