ADC is the absolute worst role to get filled to. Support you just gotta poke and use your meat to block. Losing an ADC as support is much easier to deal with, selling your support item to CS and get straight damage/tank can surprise the laners when your jungler ganks, and if you feed you're still a support so you'll be useful even if your KDA is garbage.
When you're ADC and you lose a support, the enemy laners will hunt you down and kill you until you're useless. Either your jungler ganks and spends half their time bot, or they let you die endlessly. If you're against a tank support (or Karma/Zilean) there's pretty much nothing you can do to live/cs. If you're against a carry support or supporty support (eg. Brand/Annie or Nami/Soraka) you can possibly get them into a cocky turret dive or get kills through ganks. You should have an EXP lead too, so you should tell your jungler to gank when you ping you're nearly level 6. You will still have an advantage 2v2.
If you're extremely lucky, you picked Varus and you can diddle people regardless. CSing is still risky. Ezreal is not as good, having to get out of the way of minions to hit your Q almost guarantees you lose a trade or you're too far and you lose cs instead.
No matter how you play you are at a disadvantage, you just have to think it out, you're Diamond so you can come up with better solutions than I can. I just play bot lane 90% of the time (support) and know how it goes, but don't have the mechanics or memory to check map, pay attention to ability CDs, know how long I have to make a wave hit tower, are 3 inhibs > baron buff? etc...
Either way, rare enough it shouldn't effect your overall rank over the course of the season. Might happen like 5 more times, oh well. Only truly matters when ranked rewards come out amirite.
