Drums only, I would say. He's played on a lot of records that Watt has produced.
Since Watt is often credited as musician (guitar, bass, keyboards etc.) and composer in addition to producing, I wonder if he might have contributed something in those areas as well.
The whole affair creates the impression of bringing in the big guns to ensure a hit record. Or at least to charm a label into signing him, I guess (if that hasn't happened yet). Not a move I expected (or hoped for) but it makes sense for him.
They didn’t know Watt would be a Grammy winner when they picked him but he produced
2 of the 5 most streamed albums of 2020 (Spotify).
I'm not familiar with most of his work, I think I've only heard the Miley Cyrus record and the Ozzy album he did, so I can't really imagine what to expect from this collaboration but I'm very relieved and excited (as well as a bit shocked) that Morrissey has been working on new material and that's the most important thing for now.