Grayson watched the 25-year-old grab his 30th and 31st goals of the season in the 2-0 win over MK Dons to become the first United player to achieve such a feat since Lee Chapman 18 years ago.
"He is under contract to us and I am pleased he is our player," said Grayson.
"To score 31 goals in a season is a real achievement at any level and he could have had a hat-trick, missing an easier chance when through one-on-one.
"It was a very good team performance from the first minute to the last. Jermaine scored two very different goals - one instinctive and the other requiring an excellent finish after a good move by the team.
"That was our eighth successive home win and that hasn't been done for 18 years, so the players have to feel very proud of themselves. There have been a lot of very good players here in that time."