“He has a five-year deal here, so there is a good chance, if he keeps scoring like this, that he can be a legend.
“He has a big opportunity to really make a mark in Celtic’s history.”
The Norwegian also praised the 25-year-old international striker’s application in the past year.
He said: “It is a message to everybody in life, if you want to change and you want to sacrifice to become the player you can be, then everything is possible and he has done that.
“Now he is kicking on and scoring because he is prepared for every match.”
Griffiths was booked by referee Bobby Madden for taking his top off after scoring his first goal to reveal a t-shirt with the number 50 emblazoned on the front, but he claimed it was “a million per cent” worth it.
He said: “It was an added incentive to go and get that goal and I knew when the chance came as long as I put it on target I would score.
“I texted Stevie the kit man in the morning to get a t-shirt made up for me and Scott Brown said I should have had 51 under it. Two t-shirts, but I might have looked a little bit chunky.
“To get 50 goals for any club is a special achievement, but to get 50 at a massive club like this is a joy.
United could be subject to an SFA investigation after two smoke bombs were left off in a section housing home supporters before the game.
The incidents occurred in the week that Celtic banned three supporters whom they believe set off pyrotechnics before and during the Scottish Cup win over Stranraer at Stair Park last Sunday.