Jamieson, Fausset and Brown on Isaiah 42:21

21. his righteousness--not His people's, but His own; Is 42:24 shows that they had no righteousness (Is 45:24; 59:16). God is well pleased with His Son ("in whom My soul delighteth," Is 42:1), "who fulfils all righteousness" (Mt 3:15) for them, and with them for His sake (compare Is 42:6; Psa 71:16, 19; Mt 5:17; Ro 10:3, 4; Php 3:9). Perhaps in God's "righteousness" here is included His faithfulness to His promises given to Israel's forefathers [Rosenmuller]; because of this He is well pleased with Israel, even though displeased with their sin, which He here reproves; but that promise could only be based on the righteousness of Messiah, the promised seed, which is God's righteousness.