Hartis has been to watch the youngster on numerous occasions, and says club officials are very impressed by the development of Henderson.
“Yeah I have been to see Dean a few times, along with the rest of the staff at the club, we keep regular tabs on him and we go to watch him regularly," Hartis told manutd.com.
"I happened to see him play against Liverpool at Anfield. So I am really, really pleased with how Dean is progressing. It is a big jump to go from the Championship to the Premier League, but he has shown that he can cope with that level of football at Sheffield United.
"He is an exciting prospect for us, moving forward as a club, so we are delighted to have him as our goalkeeping pool really."