Now for the real answer...
How loose is loose? It's a vague term, and you speak in generalities. With the engine off, turn the crank by hand one complete turn and then check the deflection of the tension side of the belt (the side you're worried about). Deflection shouldn't be more than a few mm. Now... there will be minor slack on the back side of the belt (the side facing the firewall), and if that slack is taken up by the cam turning on it's own (it'll do this due to pressure on the cam lobes from the rocker arms), the slack will be transferred to the tension side.
If the timing belt skips a tooth on the cam or crank gear, your valve and ignition timing will be retarded 9° (9° of timing per tooth). Refer to your shop manual that you should have regarding setting the timing properly. If it skips one tooth, which is most likely what happened if your engine is running rough, but runs, no damage has occured. If it skips more than one tooth, you could have bent some valves. however... on the few D series engines I've built, I've tested the "interference" theory by positioning a valve so it's at the top of the lobe, and then SLOWLY rotating the crank a few turns (with the timing belt off). Nothing happened. So, I'm going to go out on a limb here and guess that even if the timing belt breaks, nothing would happen. However, it's never happened to me, so I can't say for sure.