Thanks. Like 老土 said the skills can be grouped into two categori

Transferable and non-transferable. He has a lot non-transferable skill, like he knows a lot of people and their histories from my previous company.... In his prior job, he did a lot data support and has not been a functional analyst for long time, maybe never been. He helped me run report and rearrange the data which a two-year analyst can do. 

Like my friend said, my boss would fire him a month ago if I was not a key person in the team and my boss didn't want to make me uncomfortable.

I thought of downgrade. However, different people may have different reaction. A friend of mine had this experience and he thought it was even bigger humiliation.