As recently as on Sunday, the 33-year-old finished in 13th place in the skiathlon.
Afterwards, she gave positive reports about her foot, which she injured during the Tour de Ski and which has kept her from competing in classical style a few times since then.
But now that the start lists have been released for today's 10km, the American is missing. The USA has only two skiers in the race, Alayna Sonnesyn and Rosie Brennan.
USA's press officer, Leann Bentley, says that Diggins is saving herself for the rest of the World Championship in Trondheim.
There is no danger for the rest of the championship, she says to Norwegian Dagbladet according to NTB.
Late last night, World Cup leader Diggins wrote on Instagram that she was "ready for everything because I believe so much in this team" - but now she won't be starting in today's competition.
The women's 10 kilometers in classical style will be decided with a start at 15:30, the men's race starts at 13:00.
