The Monaco striker has been linked with a move to Chelsea after an impressive season which has also seen him make his France debut
Diego Costa has described reported Chelsea target Kylian Mbappe as a 'phenomenon' and believes the Monaco teenager is a 'killer in front of goal'.
Mbappe has been in stunning form in recent weeks and became the youngest player to make his France debut in over 60 years on Saturday when featuring against Luxembourg.
The striker has been linked with a string of Europe's leading clubs, including Chelsea, and current Blues forward Costa has been impressed by the 18-year-old.