Gareth Southgate said Harry Maguire's lack of playing time makes it difficult to guarantee he will start games for England while the manager also had concerns over Ivan Toney's rehabilitation during his eight-month ban from football.
Maguire, who became the world's most expensive defender when Manchester United signed him for 80 million pounds ($100.97 million), is no longer a first-choice centre back for his club despite being named the team's captain.
He has fallen down the pecking order behind Lisandro Martinez, Raphael Varane and Victor Lindelof, with even left back Luke Shaw having played in central defence, leaving Maguire with just seven starts in the league this season.
"Inevitably it's not a situation that can continue forever," said Southgate, who has handed Maguire over 50 England caps and included him in the squad for next month's Euro 2024 qualifiers.
"We've got - in Marc Guehi, Lewis Dunk, Tyrone Mings - players who are playing well without the same experiences of club European football or international football. But we've got to keep searching for competition in that area of the pitch."
Southgate was also concerned that Brentford forward Toney's punishment for breaching the Football Association's betting rules had left him without the support he needs for rehabilitation.
"What bothers me is we've got to look after people. He's injured at the moment, what does he do about getting fit?" Southgate said.
"I don't like the idea that we just leave somebody so they are not allowed to be a part of the football community, I don't think that's how we should work. I don't think that's how the best rehabilitation programs would work."