The problem is that Netty 4.1 is
not fully backward compatible with 4.0. We can recompile Play 2.5 against Netty 4.1, but we don't know if that will cause issues with Play apps, modules, or other libraries used by Play applications that are based on Netty 4.0. We generally do not like to break binary compatibility in a minor version (note: Play versions are epoch.major.minor) without a good reason.
That said, it should be possible to create a fork of play-netty-server that uses netty 4.1, and use that with Play 2.5 without changing any of the other Play dependencies. We'll need to do the upgrade for Play 2.6 anyway, so the best route would probably be to submit a PR to master with the changes first, then create a custom Netty server project based on that to use with Play 2.5.