Why is it necessary for black to defend e5 pawn after 1. e4 e5 2. Nf3? Why is it necessary for black to defend the e5 pawn after 1. e4 e5 2. Nf3? What do you think of 1. e4 e5 2. Nf3 a6 3. Nxe5 d6? Now white should move his knight to c4, d3 or f3 and black can develop more pieces.
It's not necessary; the solid Petrov's Defence continues with the counterattack 2... Nf6. There's also a less stable option in 2... f5, the Latvian Gambit but it rarely sees play at the top level.

Otherwise, the real problem for Black is not the knight on e5. It's the fact that you're giving away a center pawn without any compensation whatsoever. If you want to develop pieces, you're even better off playing 2... Bc5 than 2... a6.