Southgate ahead of Monday's game said he was willing to stake his "reputation" on the Manchester United captain, only to be repaid with two costly errors which gifted Germany two goals in a 3-3 draw at Wembley.
The Three Lions boss said he will continue to back his players, though added a late warning by suggesting an even bigger drop off in form could see his mind changed.
Speaking after the game, Southgate said: "I know everyone will focus on Harry but there were some very important moments that Harry delivered in the two matches.
"In these moments we have to back our best and most experienced players unless it's untenable."
Having given the ball away to Jamal Musiala, Maguire tried to win it back but hacked down Musiala and a penalty was eventually awarded after a VAR intervention.
For Germany's second, Maguire lost possession in the build-up to Havertz firing home from distance.
Southgate added to Channel 4: "I think as a group they've really come together this week, they've grown. This week we've learned a huge amount.
"A couple of errors have cost us the goals but they played with terrific spirit and a belief that we haven't shown in the last few games."