The 25-year-old striker arrived at the Stadium of Light in the summer from Wolves for a fee in the region of £14 million and has already bagged 10 goals in 22 Premier League appearances for Martin O'Neill's side, over a third of the Black Cats' team total.
The Scot netted twice during the club's 3-2 win away against Wigan on Saturday, with Mignolet particularly impressed by Fletcher's finishing ability.
|40/1||Steven Fletcher is 40/1 with Bet365 to be the Premier League's top goalscorer|
"He's got a very good finish – you can't leave Fletch on the ball inside the box because he is lethal. We have seen that in training and he's doing it on the pitch.
"That's 10 goals now for a very good striker – one of the best [strikers] I've played with."
Sunderlandcurrently sit 11th in the Premier League table and next face an in-form Swansea City side at home on January 29.