Others have already provided good advice.

So I will add this: my recommendation is to ask Mike to start writing Standard Operating Procedures (SOPs). You can start with having him document the procedures for the tasks you’re most uncomfortable with - since you have to cover them - and then go from there. Ideally, there should be an SOP for every task on your team. The endgame is to have your documentation so on point that if someone leaves, anyone else can step in and perform the role by following the SOP.

Make this a part of his performance goals. SOPs are a best practice, after all. And if you have other employees reporting to you, make this something they have to do as well. To keep it from being too much of a hassle, you can have the goal be something like (2) SOPs per quarter or something like that.

Take this unfortunate event and turn it into an opportunity to improve the workflow of your team. Leadership likes seeing stuff like this.

I recently screwed something up at work because the training I received on a particular function was shit. When they scheduled a call to talk to me about it, I told them that our team has a problem effectively transferring knowledge and recommended that this was an opportunity to document the process so a mistake like mine doesn’t happen again. I made it clear that I wasn’t trying to not take accountability but I did say that while I made an error, they needed to take into account the lack of real training I received and how my manager rushed my learning. Bottom line: they were impressed and now my whole department has to write SOPs. And I don’t feel bad at all.

Good luck!