The Scotland captain was sidelined for nearly all United's Premier League title run-in after being struck down by a virus which triggered significant weight loss.
Although Sir Alex Ferguson spoke optimistically about Fletcher's chances of making his squad for the Champions League final, eventually he failed even to make the bench for the 3-1 defeat by Barcelona - his second final heartbreak after he missed the 2009 showpiece between the same two teams due to suspension.
It was envisaged Fletcher would have fully recovered during the summer and he would be on Monday's flight across the Atlantic.
However, when United confirmed their travelling party on Saturday afternoon, Fletcher was not in it.
Instead the 26-year-old will remain in Manchester, spared the accumulation of travelling time to spend the next three weeks trying to get himself right for the new campaign.
If all goes well, Fletcher could be involved in Paul Scholes' testimonial against New York Cosmos on August 5, or the Community Shield showdown against Manchester City two days' later.
But Fletcher knows if he fails to play a significant amount over the course of those two games, the chances of him being involved in the Premier League season opener at West Brom will be slim.
A United spokeswoman said: "Darren will remain at Carrington to undergo an individualised conditioning programme given he missed a large proportion of last season.
"This will help him catch up fitness-wise to the levels of the rest of the team.
"We tried to accelerate things but we want to give him the best opportunity to be fully fit for the new season. We feel a reduction in the amount of time Darren spends travelling will help him achieve this."
With Fletcher missing, Ferguson will take an initial squad of 21 with him on Monday.
Of his new signings, only England international Young will be on board, having taken the number 18 shirt vacated by newly-retired Paul Scholes.
David de Gea and Phil Jones, who will wear one and four respectively, have both been given an extended break and will join later in the tour along with Tom Cleverley, Danny Welbeck, Javier Hernandez and Chris Smalling, who signed a new five-year contract yesterday.
Of his experienced players, Darron Gibson and Tomasz Kuszczak miss out, the clearest hint yet that neither has a United future.
None of the Red Devils' Youth Cup-winning squad make the journey but Federico Macheda and Mame Biram Diouf will be there after spending last season on loan, whilst Nani and Dimitar Berbatov will also be involved despite speculation linking them to summer moves away from Old Trafford.
United open their tour against New England Revolution in Boston on Wednesday before heading to Seattle, Chicago, New Jersey and Washington.