The Three Lions stretched their unbeaten run to 10 games with a comfortable 2-0 victory over Costa Rica at Elland Road.
“We have real competition for places,” said Southgate, who made 10 changes to the side that had beaten Nigeria at Wembley.
“If I look at the three weeks we have had I could not be happier with the preparation; physically, tactically and the togetherness of the group.
“There are some serious decisions to take but in a good way. I have got 12 that are going to be tough to leave out.”
The only previous meeting between these two countries had come with England, already out of the 2014 World Cup after losing their first two group games, at a low ebb.
Four years on, however, it was the Three Lions who looked much more likely to make an impact in Russia than a side who reached the last eight in Brazil before bowing out to Holland on penalties. Rashford was the standout performer for the hosts, to the delight of Southgate.
“The goal was world class,” added the Three Lions chief. “I felt at the end of the season he was trying too hard but the big thing is he enjoyed himself.
“He played with a real swagger. He is a Manchester United player and got booed at the start but cheered off at the end. That tells you about his performance.”
England’s first victory in three visits to Elland Road proved a good advert for taking more games on the road in the future.
Not only did the flags handed out free to all supporters provide a rousing send-off for the home players but the atmosphere was lively throughout.
“It was brilliant,” added Southgate. “And fantastic to see so many youngsters line the route in. That was a huge lift.
“The atmosphere within the ground was also brilliant. Little bit of good natured jesting with the Manchester United lads. Booed in but cheered out.”
Report and analysis: Page 24