England must beat Croatia by two goals on Wednesday and hope Switzerland lose to Portugal to reach the quarter-finals of Euro 2021 this summer.
They only had one shot on target – none in open play – in their first defeats against Switzerland and Portugal in Group D.
Since being a finalist at the 2009 European Championship, England have only qualified once in the squad – under Boothroyd in 2017.
Jude Bellingham, Mason Mount, Jadon Sancho, Phil Foden and Trent Alexander-Arnold could all have been included in the Slovenia squad, but Boothroyd insisted the senior squad remains the priority.
“I know that the work of the senior men has been called impossible work, but the U21s are totally impossible work,” he said. “The reason is the number of players that have to be produced for the senior team and we are expected to win.
“There’s a reason we haven’t won this in 37 years and why we haven’t progressed as much as we thought, because our main focus is to get the players through to Gareth. [Southgate].
“Can you imagine a reserve team manager knocking on the door of a senior manager of a club and saying” listen to the boss, I think today you should let me have the best player to win? the reserve championship “. It just wouldn’t happen. .
“The only team that needs to win is the senior team. Everywhere else, victory is part of it, drawing is part of it and adversity is part of it.
“The reason I say it’s totally impossible work is because new players come in, young players come in and the wheel starts all over again.
“My job is not to tell Gareth we should have Jude, Phil or Mason, this is the senior team and always will be.
“If somebody is doing this job and hiding players and not allowing them to move forward, then the system is broken. I don’t want to be the whistleblower to defend myself. These are facts, but that’s what work is. ”
Boothroyd has confirmed that Callum Hudson-Odoi suffered a shoulder injury and returned to Chelsea for further evaluation.
The 20-year-old played 90 minutes in the 1-0 loss to Switzerland but missed the team of the day for the 2-0 loss to Portugal on Sunday and Boothroyd confirmed the omission was due to a injury he suffered during the match. game development.
“Callum is definitely out and he’s returned to his club, Emile Smith Rowe is also very doubtful,” Boothroyd said.
“We are waiting to hear from a few players, we give everyone as much time as possible.
“We have a couple of bumps and bruises. I’m going to use the team and I have some good players who haven’t played yet. ”
Chelsea midfielder Conor Gallagher, on loan at West Brom, is set to start in Slovenia.