Chip MultiProcessors (CMPs) will have dark silicon or frequently deactivated areas in a chip, as technology continues to scale down, due to power dissipation. In this work we estimate the influences of deactivated cores on performance of network-on-chips (NoCs). Even when a chip has a two-dimensional mesh topology, a deactivated core that includes an on-chip router makes topology irregular. We thus assume that a topology-agnostic deadlock-free routing is used with a moderate number of virtual channels in such CMPs. Thorough cycle-accurate network simulations of a 2-D mesh NoC, we found that (1) indeed a deactivated core degrades the performance to some extent in terms of throughput, but (2) latency is not increased or even reduced when a deactivated core is located in the corner of a mesh. Hence, we recommend choosing a corner core for deactivation to maintain the performance of NoCs.