Pork loin should be called pork lean really, the modern hog has not much fat left in its meat, especially the loin.
Not much you can do about it after it's cooked to such a high temperature, unless maybe cube and stew it.

Before baking it in the oven, there are a few steps you can take:

• Pre-heat the oven at high temp at first to form a crust on the outside that will seal in the juices, or sear it on all sides in a grill or pan first, then bake at reduced heat with a little braising liquid.
• Regularly spoon the juices over the roast while cooking.
• Don't salt the meat before cooking, only when the roast is done. The salt draws out the juices.
• Marinate it before cooking rather than a dry rub, maybe even use a turkey baster.
• Butterfly it, fill it with a moist stuffing that contains butter, then reroll it into a roast.
• Cover it with bacon or, as all roasts (pork, beef, veal) are prepared in France, with a thin sheet of backfat.
  Butchers have a special machine that will slice fatback (without the rind) into one millimeter thin sheets that are used to wrap roasts or small birds, to line terrines or put at the bottom of a crockpot or pan before nonstick existed...

(pork loin roast barded with fatback.jpg)

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