From his official website maceoparker.com:

News updates for March 2004
European Tour Dates Cancelled

We are sorry to have to announce that Maceo's European Spring Tour has been cancelled due to ill health.

Thanks for your many messages on this matter.

We know many people are concerned and want to let you know that while this is not something to be worried about it has, however, been necessary to cancel the Spring tour.

We will keep you informed.

Maybe this is why he was replaced for the Ellen Degeneres appearance. Let's wish him a speedy recovery .