I missed this thread when nit was created, but looking at the other responses, I doubt I could have added anything anyway.
Moving to the current situation. You must write to your boss, outlining your concerns, and ideally, making suggestions for some resolution. Having been forced to have time away, the silver lining is all parties probably realise that your not having cover isn't a decent long term option. Or did someone else play the hero/martyr in your absence?
But. Bottom line is that unless you are a complete subject matter expert, in a highly confidential, specialist technical area, there should be cover, for succession planning at least. What would happen if you tripped over tomorrow and broke an ankle? That could mean6-8 weeks away from work, which is probably an intolerable gap in the work you do.
I would suggest you ask your manager to share his thoughts for how planned, or unplanned a senses will be covered, so that you can both be assured the cover is appropriate, and any required up-skill img is delivered to the person who would cover for you. Obviously, you can reiterate the overload situation you find yourself in, and ask for support; even if one or two days a week.
When I used to have these challenges from time to time, from direct reports, I used to ask them to perform an analysis of what they actually did, and how they spent their time, over a couple of weeks. Moto be fair, that was enlightening to both parties. How much time can't you account for? With that evidence, it's hard to push back. Bizarrely though, in my lasts employed role, I idid have someone who was a true single point of failure; which he liked. Meh liked the control. I hated that I could bein the lurch at any moment; given the guy had a bad back, that sometimes got suddenly very bad, needing time off, when the going got tough! In the ends, I had to threaten him with a disciplinary process if he could not show me documented evidence, and demonstrate, his own succession plan. The man was mildly bonkers, in that he could ever have holiday at the start of any month, then used to complain about it!!
So, please try to adopt a positive approach. Putting it in writing is important for you, should nothing transpire and you need more unplanned time off. Many companies have tight absence policies, and you've already had one caused, at least in part, by your work situation. You have to show you are trying to be part of the solution, not the problem. So, even if you discuss the matter, drop him an email, copyyourself in( then you know it has gone and been delivered. I hate to say it, but this is just protective planning for yourself. I wouldn't necessarily involve ahR right now, until you know what his response is.
Good luck with it.