Hey, welcome to the game!
1. I think your Celtic game is wrecked. Sorry.
In the "Too late to help" category, note that you can at least minimize the chances of anything game-wrecking by using the "Rolling Auto-Save" option. This will create several backup copies of your save game as you go along. For example, if my save game is "Amaroq.fm", it'll make files named "AmaroqV2.fm" and "AmaroqV3.fm". I
strongly recommend that you turn that feature on in future!
If you've
already got that option on, you're a lucky man - you should be able to load one of the older versions of your save, before the fixture problem occurs, and proceed from there.
2. Installing a patch is a super simple task. Download the patch. Run the patch. Follow the directions.