I don't have an issue with Mo, it's ok to be nervous and I am sure there will be mistakes. But the fact is that every starter had their first season starting in professional ranks.
I try not to worry about it too much. Just line up and play ball. I am sure the Rams would have liked to keep McLeod but they obviously had a price tag in mind and figured they had a replacement on the roster and that's why they didn't go out and sign a big name in free agency.
When it comes to defensive players I tend to trust this staff much more than I do with offensive players. If they believe Mo can handle it then I trust their decision.