Grant was jeered by some Addicks supporters on his return to The Valley on December 10 and responded by way of two goals in the Terriers' emphatic 4-0 victory over the Londoners.
Cowley, whose side have moved five points clear of the Championship relegation zone, said: "He got a bit of stick at Charlton which I thought was unfair as he came through the academy and Charlton Athletic supporters should be proud of him.
"To get the criticism he did personally, I thought it was unfair.
"He was excellent. The thing with Karlan is that he has come off the back of an injury and we had to rush him back last Saturday because of Fraizer's (Campbell's) illness. But he always finishes games and is always able to do ninety minutes.
Credit to him as he has a real appetite and a hunger to that and not all attacking players can do that physically and mentally. But he can and he got his rewards for all his endeavour and hard work late on."
Meanwhile, Cowley has confirmed that midfielder Andy King should return to training on Monday, while Jonathan Hogg is due to a see a specialist regarding his knee injury.
"Kingy's is a really low level and he should be on the grass on Monday. We just did not want to risk him.," the Town chief added.
"Hoggy will see a specialist early in the week and we have got everything crossed. He is moving and he has had a medial (ligament injury) before and this is a lateral and he had a knee construction on that side and it is certainly not as bad as that.
"It is (also) not as bad as the medial one which kept him out for eight weeks (before). We are hoping it will be sooner rather than later. But he is an brace at the moment and we are just taking necessary measures."