Without the Italians, the Germans will need to field three new corps.
New Units in the prewar army
They could do this quickly by using the six Regular infantry brigades in five reserve corps (V, VI, VII, IX and XIV Reserve Corps) and one in the independent 33 Reserve Division.
Those six brigades added to six new brigades would provide infantry for the six new divisions.
They would need to add three new Jäger Battalions and six new pioneer battalions along with the other support units for the six divisions and three corps.
On the artillery side they would need 36 * 7.7 cm Guns and 36 * 10.5 cm Howitzers for each of the six new divisions, and 16 * 15.5 cm Heavy Howitzers for each of the three new corps.
Manpower
Each brigade has 7,000 men. If two existing infantry brigades get used for each corps, they will need 44,000 - 14,000 = 30,000 new men per corps for a total of 90,000 new regular troops.
They could then either:
- Create 6 new Reserve brigades, 42,000 men to replace those units in the reserve divisions or
- Use 6 Ersatz brigades from the 6 ersatz divisions, which would eliminate 3 Ersatz Divisions or
- Use 6 Ersatz brigades from the LW Divisions/Corps, which would eliminate 3 LW Division equivalents or
- Use 6 LW brigades which have lower manpower and less artillery to backfill the Reserve divsions.
90,000 new regulars plus 42,000 new reservists = 132,000 new troops total.