If what was left was only byssal thread the clam should be fine. If the entire byssal gland or foot was torn out the clam will show signs of stress and die. To me it doesn't sound like what was torn was anything more than byssal thread. Still I would find a place for the clam and leave it. These clams can get stressed from jumping off, bailing out or being moved too many times. If the clam looked good and was happy in the rocks I'd move it back and maybe place a few small pieces of
rock rubble around it so it can reattach and then you can remove the rubble.
If the clam has extended it's mantle since the thread was torn out it should be ok. I would still be careful and watch it and make sure it doesn't get anything picking at the opening til it heals.
HTH...