The Scotland right-back reckons Black Cats striker Fletcher, a friend and a former team-mate, will pose a massive threat today.
Whittaker, 28, a summer signing from Rangers, said: "We were in the same team together at Hibs, although he was a couple of years younger.
"I have played a lot of games with him, enough to know just how good he is. Even then he nearly always popped up with a goal or two. That's why I was so delighted to see him back wearing the Scotland jersey recently because we have missed him.
"Since he moved to Sunderland he has shown just what a good player he is. He is great in the air, that's probably his greatest strength. He gets a lot of his goals that way but he's got a great left foot as well.
"He always had a lot of talent so I'm not surprised by his success."