Not very. If you see blood, then it's serious. They like to swim over/under things, and rip their tails often. It will grow back together, just give it some time.
Fantail goldfish ripped tail is not something to be overly concerned with unless he/she is housed with other fish that tend to pick at the damaged tail. Fish often lose fins due to a condition called fin rot, and also damage them in aquarium life(plastic plants being the enemy). The fish will most likely adapt to his new situation.
The goldfish should be just fine as long as the water is kept really clean in order to allow the tail to heal. However if there is discoloration around edges of the tear, there may be cause for worry.
If your fantail's tail was ripped by another fish in the tank it is probably a sign that the fantail is not able to compete with other fish in the tank for food. I would watch the fish carefully to make sure that they aren't nipping at each other. If the behavior is bad, more the fantail.
