Because of interwell interference, the completion and production of infill wells in unconventional reservoirs often change established production profiles for parent wells and lead to infill-well production lower than expected. Parent-well injection has been used in some fields in an attempt to reduce interwell interference. However, mixed responses were received from these attempts, and few modeling studies have been presented to investigate the mechanisms of the mixed responses. This study investigates the effects of subsequent injection in parent wells with legacy production on interwell interference using a data set from Eagle Ford Shale. A numerical-modeling work flow is presented for the characterization of poroelastic behaviors of multiphase-fluid diffusivity and rock deformation using the finite-element method and multifracture propagation using the displacement discontinuity method. It solves for the spatial-temporal evolutions of pore pressure and in-situ stress because of parent-well production and injection and models the fracture propagation during infill-well completion on the basis of updated heterogeneous in-situ stresses. Thus, the approach obtains the interwell fracture network comprising parent-well fractures and fractures from infill-well completion and captures fracture hits, which are necessary for the analysis of the injection effectiveness. Numerical results indicate that subsequent injections in parent wells make infill-well fractures grow more transversely, denoting improved completion qualities of infill wells. Also, the required subsequent injection volume leading to transverse infill-well fractures is positively correlated with the volume of legacy production in parent wells. In addition to subsequent injection volume, locations of perforation clusters along the infill well are another key parameter affecting the associated interwell interference. Results show that it is easier to generate fracture hits after infill-well completion, when perforation-cluster locations along the infill wellbore are identical to those along parent wellbores. In contrast, certain infill-wellbore perforation-cluster locations different from those in parent wellbores guarantee transverse infill-well fractures and avoid fracture hits during/after infill-well completion. On the basis of the numerical results in this specific study, when infill-well perforation cluster locations are properly placed, the volume of parent-well subsequent injection should be at least 76.9% of the total depleted liquid volume during the legacy production of parent wells for subsequent injection to be effective in avoiding fracture hits. This value is on a case-by-case basis and should not be generalized. The contribution of this work lies in its analyses of the mixed performance by parent-well subsequent injection in the reduction of interwell interference using a reservoir-geomechanics/fracturing modeling work flow.