He was playing before Penn got hurt it is just that after Penn got hurt his minutes went way up and he was asked to be their main scorer. Penn has missed several games this year with injury as well.
Also, Hemphill hasn't looked like the same player after he came back from injury and started at the start of the season but has been in and out because of injury issues and that is why he is coming off the bench. But he is basically playing his normal minutes when he is available.