The Three Lions captain was frustrated for large spells at Wembley Stadium by the visitors and was eventually replaced with 16 minutes remaining.
And Southgate, whose side face Czech Republic in the final Group D game with the winner destined to win the group, explained his decision was motivated by trying to force a victory.
“I think it was a frustrating night, we know we can play better,” Southgate told ITV Sport. “You have to give Scotland credit. We didn’t do enough to win the game. If you can’t win a game in a tournament, you make sure you don’t lose it.
“I felt that we needed a few more runs in behind, I felt Marcus would give us that energy. We had to try and get the win at that point.
“We have to make these decisions based on what we see. I think the whole team, we can look at ourselves, it starts with me, we’ve got to do better. Not enough attempts on goal.
“I think we’re still working those patterns, we’re playing at a very high level of games, huge intensity, to find space tonight was really difficult. Scotland defended really well.
“I think I know exactly where this team is, it’s a very young team, some of them won’t have experienced that intensity and pressure, that’s a huge learning for them. We have to improve for our next game.”
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