"Statistically last year we were picking from a pool in the Premier League of 33 percent English players, this year we are down to 30 - and [among] the top six teams we are on a downward curve", he said after announcing his squad for next month's global games with Spain and Switzerland.
The Three Lions squad report to England's training base St. George's Park on Monday September 3.
And the Evening Standard say that United have been preparing to offer Shaw a new deal even before his early season form.
Shaw has not played for England since March 2017.
"There's a point where I don't need to know what those guys can do as we're looking to bring younger players through".
Southgate explained that he spoke with Young as well as Leicester striker Jamie Vardy and Chelsea centre-back Gary Cahill about their involvement in the national team, with all three taking a step back to give younger players a chance but still making themselves availalbe if needed. "Tactically he has a better understanding and we are very happy for him".
The 23-year-old has backed United to respond to their stuttering start to the season as he dismissed suggestions that Mourinho has lost the dressing room.
"We're on the back of a successful tournament", said Southgate.
The 31-year-old believes the time is right for younger players to be given an extended opportunity, although he could still add to his 26 caps in the event of an injury crisis.
"He had that balance and consistency when the team didn't win, especially against Brighton he kept that consistency".
"We wanted some continuity (but) we won't stand still". At the minute that's a result of the hard work, he and the coaches have done.
Mourinho was also asked about the prospect of Anthony Martial signing at new contract.
"Martial - we have to wait for it to be official", Mourinho told reporters on Friday.
That means the likes of Ruben Loftus-Cheek and Fabian Delph have retained their places in the squad despite not featuring too often for their respective club sides so far.