The affix ap comes after the verbs 1. vyadh 'to hurt or to pierce' 2. jap 'to mutter' when no upasarga is in composition with them.,
This debars घञ्. As व्यधः 'wound'; जपः 'muttering'.
Why do we say 'when upasarga-less'? Observe आव्याघा, उपजाया ।,
व्यध-जपोर् अनुपसर्गे
Adhyāya 3 · Pāda 3 · Rule 61