The West Yorkshire outfit will take on Fulham at Elland Road in the Championship this evening.
“Massive threat, they are a massive threat to anyone in the Championship,” Evans told BBC Radio Leeds about McCormack and Dembele.
“Ross McCormack has been subjected to multimillion pound offers at his time at Fulham, and the boy Dembele is a special player, so my back-five, including my goalkeeper, need to be on top of their game."
McCormack is one of the best strikers in the Championship and has been in good form so far this season.
The 29-year-old Scotland international, who played for Leeds from 2010 until 2014, has scored 16 goals and created 57 chances in 30 league matches for Fulham so far this campaign.
The former Rangers man has a pass accuracy of 80% and a shot accuracy of 67%, and has won one header, 20 dribbles and three tackles.
Dembele, who is just 19 years of age, has been a revelation this season and is one of the best young players in the Championship at the moment.
The France Under-20 international forward has scored 10 goals and created 24 chances in 29 league games so far this campaign.
The teenager has a pass accuracy of 69% and a shot accuracy of 51%, and has won 102 headers, 20 dribbles and 11 tackles.