Erik ten Hag's squad will return on a staggered basis over the next few weeks, with those on international duty set to come back later.
The initial group of players due back reported for medical testing this morning, followed by a first training session in the afternoon.
The Reds who returned on Monday included Andre Onana, Victor Lindelof, Harry Maguire, Aaron Wan-Bissaka, Casemiro, Mason Mount, Antony and Marcus Rashford.
Redevelopment at Carrington means the players are using different facilities this season, as work gets under way to modernise their usual building elsewhere on the site.
All our teams – men’s, women’s and Academy – will experience changes to their usual training environment this season as the project progresses, while maintaining access to high-quality facilities and pitches throughout.