The Three Lions international was called up to Gareth Southgate's squad for the UEFA Nations League in June after starring during his loan spell at Crystal Palace in 2021-22.
Gallagher had started to force his way into the Blues lineup before the sacking of Thomas Tuchel, starting the last three Premier League games in which he has been available.
The 22-year-old received a red card in one of those games against Leicester City back on August 27, and Southgate has preferred the likes of Kalvin Phillips, Jude Bellingham and James Ward-Prowse this month.
As a result, Gallagher is back in Lee Carsley's squad as the Young Lions take on Italy Under-21s and Germany Under-21s in two friendlies on September 22 and 27 respectively.
"We welcome Conor Gallagher back to the group which is another example of us having a fluid pathway across our national teams," Carsley told englandfootball.com. "I know Conor will take to the pitch with as much pride in wearing an England shirt for the U21s as he would for the seniors.
"He - like all of our players - can continue to take comfort in the fact that Gareth will always keep one eye on their progress; there is no glass ceiling."
Gallagher forms one part of a recognisable squad for the Under-21s, as Tottenham Hotspur duo Ryan Sessegnon and Oliver Skipp return alongside Sheffield United's Rhian Brewster.
Sessegnon has enjoyed a battle with Ivan Perisic for starts in the Premier League this season, although Skipp has not earned any minutes for the senior Lilywhites side following his injury problems.
"In Ryan Sessegnon, Oliver Skipp and Rhian Brewster, we also welcome back experienced players at this age group who I'm sure will play an important part - on and off the field - in our preparation for next summer," Carsley added.
There is only one player in the Young Lions' squad who has been called up for the first time in 18-year-old Manchester City defender Luke Mbete, who is spending the 2022-23 season on loan at Huddersfield Town.
"Luke is a player who has been in our system for a while, he's headed out to Huddersfield this season for his first loan move and he's a player who we see as having high potential within the England set-up. We look forward to working with him," Carsley explained.
However, there is no place in the team for Emile Smith Rowe - who also missed out on a call-up to the senior team - as the Arsenal man continues to work his way back from a groin problem.
Fellow Gunner Folarin Balogun has been selected, though, and the striker will be aiming to build on a stellar start to life on loan at Reims, where he has scored five goals and set up two more in seven Ligue 1 appearances.
ENGLAND UNDER-21S SQUAD IN FULL:
Goalkeepers: Joe Bursik (Stoke City), Josh Griffiths (Portsmouth, loan from West Bromwich Albion), James Trafford (Bolton Wanderers, loan from Manchester City)
Defenders: Max Aarons (Norwich City), Charlie Cresswell (Millwall, loan from Leeds United), Taylor Harwood-Bellis (Burnley, loan from Manchester City), Luke Mbete (Huddersfield Town, loan from Manchester City), Levi Colwill (Brighton & Hove Albion, loan from Chelsea), Ryan Sessegnon (Tottenham Hotspur), Djed Spence (Tottenham Hotspur), Luke Thomas (Leicester City)
Midfielders: Tommy Doyle (Sheffield United, loan from Manchester City), Harvey Elliott (Liverpool), Conor Gallagher (Chelsea), James Garner (Everton), Morgan Gibbs-White (Nottingham Forest), James McAtee (Sheffield United, loan from Manchester City), Jacob Ramsey (Aston Villa), Oliver Skipp (Tottenham Hotspur)
Forwards: Folarin Balogun (Stade de Reims, loan from Arsenal), Rhian Brewster (Sheffield United), Angel Gomes (Lille), Anthony Gordon (Everton), Keane Lewis-Potter (Brentford), Cole Palmer (Manchester City)