Help advice needed !!!Watch
1) Put in as much work as you can to know the concert inside out, ask if you can have any other rehearsals throughout the week and reassure your bandmates you are ready to do the concert.
2) give this one a miss - there will be other concerts other than this one and I'm sure they can fix something if your instrument isn't present.
I would go for option 1 personally - in a band, every role is integral - you can't let them down (or the fans!)