The WRU announced on Thursday that Faletau needed surgery after injuring his collarbone, with no timeframe given for his return.
On Friday, the rugby star tweeted a photo from hospital next to his son, Israel.
In the post, he said: "Thank you for all of your messages of support. Disappointed with how things have turned out, and to miss out on the chance to represent Wales at RWC 2019 - but it wasn’t meant to be. Wishing the rest of the squad every success going forward. See you soon."
The news was a bitter blow for Wales and for Faletau himself, who's suffered a string of injury layoffs over the last few seasons. In December 2017 he picked up a knee problem which saw him miss part of that season's Six Nations.
In October 2018, Faletau was ruled out for 13 weeks after breaking his arm while on duty for his club Bath, before suffering an identical injury in his comeback game the next January. The two injuries saw Wales' talismanic number 8 miss that season's entire Autumn Series and Six Nations.
Wales have won their last twelve matches without Faletau, a run which saw them topple South Africa (twice), Argentina (twice) and Australia, as well as completing a Six Nations clean sweep earlier this year.
But being without such a proven operator will still be a big blow for Warren Gatland and his team ahead of the forthcoming World Cup in Japan.
In the next month, Wales face home and away warm up matches against England and Ireland. They take on Georgia in Toyota City in their opening World Cup pool match on September 23rd.