The best thing you can do is leave it. Pun intended.
Your pokemon will be gathering it’s energy to grow a new one soon, after it drops, give it 3-4 days and your Pokémon will be back to normal, with a fully grown new leaf.
The damage happens often in battle and will put your chikorita in a bit of a bad mood normally while it grows back.
“Here in the Pacific, 200 metres down, we enter an alien world… This is barreleye a fish with a transparent head filled with jelly so that it can look up through its skull.”– Sir David Attenborough
Blog 