What does he do stay on his line and let Lukaku fire a shot off, real issue is someone should be tracking runners rather than watching the ball.
I'm with Tom here because Lukaku got the ball near the by line. He can't possibly score or cause danger from there. Runners should be tracked, as you said, but it's impossible to play a perfect defensive game. There will be times when a runner loses his marker. You try to make the right decision and only come out when you can get the ball.
I suppose it's a risk that comes with playing the way Ederson plays (a bit of a sweeper keeper), and we shouldn't be too hard on him, but instances like these are very avoidable. It never LOOKED like the right decision to go for the ball there. We could see the penalty coming the moment he launched himself at Lukaku's feet. And when you do that, you're gifting a penalty to a team that hasn't created a goalscoring chance. Just a gift.
Arteta looked really angry with him last night but Guardiola seemed to reassure him that everything is okay while trying to show him some support. I see both sides. We shouldn't castigate him but I can understand some members of the coaching staff being livid with him. Margins are slim at the top level and mistakes like these could cause a difference next time around.