So I was just listening to 'Goats Head Soup' for the first time in a while and, although I was enjoying the album from the get-go, it wasn't until 'Silver Train' came on that the bass playing really caught my attention.
I googled the personnel for that song, assuming it must be someone other...
So, it looks like Mick's heart is the problem...
Mick Jagger: Heart Surgery Postponed Rolling Stones Tour – Rolling Stone
Of course, Keef copes with the news with a beer on the balcony...
What a drag it is...I guess you can exercise too much.