Explain to your manager that if an employee can merge code without reviews, he should expect breakages on regular basis.

Propose that your team can meet to talk about how to make the process better, so no breakages occur.

Mention personal responsibility and knowledge - it’s a good practice that a person who wrote the latest code changes either helps either is responsible for making the service working again, if there are issues - specifically because they might have some idea what’s broken.

If your manager will ignore it, I’d start looking.