Hernandez, whose work permit was only finalised in the last couple of days, opened the scoring in the first half, before Diame marked his debut with a goal against his former club after the break.
The manager told Sky Sports News: "The two goals, the first one in particular from [West Ham's Enner Valencia], you just have to hold your hands up, it was a wonder goal and it made for a really good night of football. It was open, end-to-end and all credit to West Ham, they played some good football today.
"If you see [Abel Hernandez] now, poor kid! He has been waiting for two weeks, he has been away in Japan and South Korea and stuck in Rome for the past couple of days waiting for his passport. He was full of life in training and that was what made my mind up, seeing the way he was, so I thought just throw him in. It was great also to see Mo Diame score, a great night for the two new signings."
"Me and Sam Allardyce are going to have a glass of beer now and say we usually don't play like that!! We are usually a lot tighter!"
Hull are back in action on Saturday against Newcastle United.