I disagree; above all else, the sprite's features should be able to be made out from regular distance, instead of having to either zoom in or get closer to the screen. The sprite is a representation, after all, so it shouldn't look like a garbage dump of pixels moving across the screen. If you're going to accentuate detail in higher resolution, the use of antialiasing and shading is what helps the most.
I accentuated his eyes just by adding two green pixels near the top corners of both sides. That's it; extra shading to the face to shape it out a bit more into the chibi head style like before, two green pixels to show his parted hair, and more black pixels to represent the sideburns. A mouth isn't necessary at all here; adding one doesn't help with identifying a face, it's more for identifying expression. Which is why in these 2D games, the mouth only shows up where expression plays an important role: Attacking, holding up items, falling down and screaming, etc. Otherwise, it can just not exist and be fine, keeping the neutral expression.
I accentuated his eyes just by adding two green pixels near the top corners of both sides. That's it; extra shading to the face to shape it out a bit more into the chibi head style like before, two green pixels to show his parted hair, and more black pixels to represent the sideburns. A mouth isn't necessary at all here; adding one doesn't help with identifying a face, it's more for identifying expression. Which is why in these 2D games, the mouth only shows up where expression plays an important role: Attacking, holding up items, falling down and screaming, etc. Otherwise, it can just not exist and be fine, keeping the neutral expression.