How much tooth needed for a crown? Is it best to pull the tooth and replace it with an implant?

I read that "... even if you have had the root canal and the post and core, BUT you do not have about 4mm of tooth left above the gumline, it is best to pull the tooth and replace it with an implant. The problem is that a small piece on the back of my tooth broke off, leaving the back of the tooth well below the gum line. Dr. says I need a painful crown lengthening. What are the chances are that this tooth can be repaired with good long term (>5 years) results?