I remember reading this book as a young adult, in the early to mid-80's and there was a particular scene that stands out in my mind where the Earth boy is stabbed in the leg by the enemy alien and the other alien boy brings him to his ship and heals him but informs the Earth boy that he has to help him capture the scary enemy alien.
If it was by Hal Clement, then Needle (1950) is by far the best match.
The basic premise is that there are two shape-shifting aliens capable of symbiosis with a wide range of hosts: a "police officer" and his target. During an interstellar pursuit, they both end up crash-landing on Earth. One of them (Hunter, the protagonist) enters symbiosis with Robert Kinnaird, and the two of them try to figure out who the host of his target is.
The leg injury you refer to was an accident, not an attack.