Look at the draft: who stands against you on the lane, who will gank you and what they will catch you with in the midgame. Think about who you can counter-pick: they have Medusa — take Anti-Mage; you are against Enigma — take Gyrocopter.
The draft decides 50% of the match. You took a hero into a draft it does not fit — you are playing with a weight on your back.