Delayed sowing of winter wheat is increasingly common in the winter wheat–summer maize rotation system of northern China, yet whether its yield penalty can be mitigated without increasing seasonal water and nitrogen (N) inputs remains unclear. A three-year field experiment was conducted to test whether stage-specific drip fertigation could mitigate yield loss in winter wheat sown 20 d late without increasing total seasonal water and N inputs. Delaying sowing by 20 d under conventional management reduced grain yield by 21.0% relative to wheat sown at the conventional date (CK). The optimized regime (DF4) achieved a yield recovery rate of 100.7%. Grain number per unit area in DF4 was partly restored but remained 5.6% lower than in CK, indicating incomplete sink recovery. Yield recovery was therefore supported by post-anthesis source compensation, evidenced by a 17.0% increase in post-anthesis dry matter accumulation relative to CK and associated with higher post-anthesis N uptake and radiation use efficiency. Although root penetration depth under DF4 remained lower than in CK, root length in the 0–40 cm soil layer was restored to the CK level. DF4 also improved water use efficiency by 11.1% relative to CK while soil water consumption was only 56.4% of that in CK. These findings indicate that stage-specific timing and placement of water-N supply through drip fertigation can mitigate yield loss in winter wheat sown 20 d late by coordinating partial sink recovery with stronger post-anthesis source compensation, without increasing seasonal water or N inputs.