"We looked through different players over the last couple of years and decided, because of the way we want to play and the type of players we want to work with, that there are some guys who didn't fit that model," Southgate told Sky Sports.
"We're into a fresh cycle of probably not going back to players who have been in before and that we've had a good look at," the former England under-21s coach added.
"So the next group that we're looking at are probably coming through our younger age groups."
Southgate has also been forced to look for fresher options after the experienced duo of defender Gary Cahill and striker Jamie Vardy ended their international careers last week.
England won the Under-20 World Cup, the Under-17 World Cup and the Under-19 European Championship last year and Southgate has continually pushed for regular game time for some of those players in the Premier League.
England will be without the injured Adam Lallana and Raheem Sterling for the Spain clash as well as the friendly against Switzerland at Leicester's King Power Stadium on Tuesday.
(Reporting by Shrivathsa Sridhar in Bengaluru; Editing by John O'Brien)
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