Sylvester Stallone has a big roster of franchises under his belt, all of which he is intimately involved in. Usually he plays at least the title character (Rocky Balboa, John Rambo) and will often also write and direct the film too. But with Creed 2, he decided to recuse himself from all his usual leadership roles. He is simply acting. The reason for this, speculated on until now, is that he thinks it's about time that he (and we) moved on from the old and brought in some new blood. Sly, interviewed by the Toronto Sun:

I was going to direct it, but I realized youth must be served. This kid (director Steven Caple Jr.) has such great vision and energy. Making this kind of film is incredibly taxing because you’re doing (the directing) and the drama. You’re doing two things. It’s at least six months shooting, maybe a year of prep, so when it’s finished, Steven will have been on this movie for two years straight.

He also talked about the difference between Rocky and Creed:

The irony is, as my life evolves, I find new things in the Rocky character. In the last one, he faced death and his own mortality. In this one, he’s finally completely alone. The only thing he has left is this young man. So he’s now part of the Creed universe. The Rocky universe is gone. It’s left in the graveyard.

Watch the Trailer for Creed 2 below: