England were beaten 1-0 at Wembley, taking a major hit in the Nations League.
Belgium's win over Iceland meant the Three Lions went from top of the group to third.
And it came about after a disastrous five-minute spell that saw Harry Maguire sent off and Kyle Walker concede a penalty.
The spot-kick may have been a soft decision but there was nothing questionable about the referee's ruling to award Maguire a red card.
It meant England were always chasing the game against the odds.
But as the match went on, Southgate seemed reluctant to throw caution to the wind and go for it.
Only with 20 minutes to go did he make changes, with Jadon Sancho, Dominic Calvert-Lewin and Jordan Henderson coming on.
However, it meant there would be no sighting of Grealish, who has been hugely impressive for Aston Villa this season.
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The decision to leave him on the bench left Redknapp dumbfounded and he said in the Sky Sports studio: "When the game is tight and you want someone to come on and do something different, I just don't think it's on Gareth's radar to bring Grealish on.
"He played so well against Wales and it's almost not done him a favour because Gareth is not going to use him right now, he just doesn't see him as the sort of player he needs. I'm surprised by that."
Alex Scott echoed Redknapp's sentiment, who insisted Southgate needs to give Grealish a chance.
She said: "He'll be going away from the England camp frustrated, thinking that when he was sitting there, he could've come on to try and be that player to create something.
"The debate will continue with him, we've all seen what he can do, we've all seen the start to the season that he's had, so when he is going to get the chance to do that?
"You take Mount off and you can bring Grealish on to open things up, he can link the play, give it to Harry Kane - he would want a player like that playing behind him and creating something."