In reply to Jack McKechnie:
Sounds like the correct assessment, and it is indeed a bitch to get better. I injured both that lumbrical and the FDP (the deep muscle in the belly of the forearm to which the ring and little finger tendons attach) 18 months ago, pulling hard on a 3 finger pocket with the little finger dropped. Dropping the little finger is actually a stronger position (hence why we tend to do it automatically in a 3 finger situation), but it means that while the ring tendon is extended the little one is contracted. This subjects both FDP and lumbrical to shearing forces, and they can tear. (MacLeod refers to this injury as the Achilles heel of the otherwise very safe open-handed approach.)
The good news is that buddy-taping little and ring fingers together prevents the tendons pulling in different directions, and so precludes the shearing action which causes the injury and any ongoing pain. After a couple of weeks I was climbing pretty freely with my fingers buddy-taped, though I did avoid pockets for a while! It was inconvenient - certain small holds and the handling of gear can be a little awkward - but it didn't have too severe an impact on my climbing. Obviously keep a keen eye on it, and be willing to let go (where possible) if it might be loaded dangerously.
The FDP takes forever to heal fully (something MacLeod and others note). After taping for 8 or 9 months I began to climb gently without, finally climbing completely without tape a year later - only to re-injure it. I'm still buddy-taping, 18 months after the original injury, although I've long since reduced the tape to a thin strip at the base of the fingers, to prevent only the most extreme tendon separation. The lumbrical can feel tweaky after a hard session (it warns me by becoming 'hot' in my hand), but the tape has successfully prevented any significant re-injury.
Julian Saunders wrote about such injuries, 'Buddy-tape, and climb as normal'. Which if you have to have an injury, is a pretty happy state of affairs - if only all injuries could be accommodated so easily..