If you don't have packs to spare then either sell him (his injury disappears when sold so it doesn't affect his sales value or attractiveness) or just let him sit it out until he's better.

As others have said nothing bad will happen and he'll get 40% condition back each day.

The only 'bad' thing that happens is he misses out on the opportunity to grow so that can be a factor if its long-term and he grows fast'ish.