Left back Shaw comes in at the expense of United team mate Ashley Young and there are also returns at the back for Liverpool's Joe Gomez and James Tarkowski of Burnley.
Lallana returns having missed out on the squad for Russia after an injury-hit season, while Southampton goalkeeper Alex McCarthy -- who has been called up twice before but never capped -- replaces the injured Nick Pope as back-up keeper.
England host Spain at Wembley on Sept. 8 in the UEFA Nations League and then take on Switzerland in a friendly in Leicester three days later.
Chelsea defender Gary Cahill and Leicester City striker Jamie Vardy had both announced on Wednesday that they were stepping away from the national team and Southgate said he had talked to 33-year-old Young about his preference to bring in younger players with an eye on Euro 2020.
"It would be unfair to say he was happy as he wants to play but he understands I want to see some young players in the shorter term," said the England manager, who was pleased to see Shaw back in the United first-team.
"He has had a difficult period in adapting to a new club and a horrendous injury. I am impressed with his physical condition and he had an extra edge in recent games when it has been going against his team, looking back to his powerful best. We felt he can add that extra impetus in wide areas," he said.
Southgate noted there were only 20 games until the next tournament but that he was also keen to keep together the core of the squad that enjoyed their run to the last four in Russia.
"We're coming off the back of a successful tournament so not a lot has changed," he said.
"With us only being three games into the (domestic) season, I felt some continuity was important. Those who were with us in the summer deserve the right to go again.
"We wanted some continuity (but) we won't stand still. We've added young players Luke Shaw and Joe Gomez.
"We're aware of the excitement around some other young players but we think it's a bit early for them."
(Reporting by Simon Evans; Editing by Toby Davis and Pritha Sarkar)
