Somewhat different to the XV he will field. This has been an issue since 2003 in that we go into tournaments with a dozen or more of a 35/36 missing through injury or other issues. The only time this didn't happen and we picked our best side was the last rugby WC.
I've been saying for ages we won't win anything until we learn to manage players work load and recovery better. Look at Ireland and France. It no coincidence that they are in top 3 sides in the world as they pick virtually the same XV throughput the year barring one of two injuries. England are continuously missing half a dozen starters and same again in terms of bench players