Games like Atalanta and Old Trafford were the defence lost their heads.
We were completely overrun by Atalanta (the disparity had some even speculating on here they were on something) from the very start when Kelleher made that save with his head.
We were ambushed by a more more athletic team, especially in midfield, and they were able to breach the back line far too easily.
Without protection infront of the defence there's very little the like of Matip could have done to comtain the bombardment.
At OT, the Quansah pass, Matip likely wouldn't have done that. Still, one ball over the top by Casemiro to the area vacated by the show 6 meant they too were able to freely run at the back line, which ended with the fella finishing top corner (very hard to see what any defender could have done to stop that).
We've been leaking easy goals all season, including when Matip was in the team, as we've had the same flaky midfield all season. 23 times now gone behind. This isn't a late collapse. The leakage has been there all season, marginally better than last season when we had no midfield protection at all. The central defence has generally been excellent. Virgil, maybe the Arsenal mix up, cant think much else. Quansah, again, switching off for the pass at OT, and the dead rubber vs West Ham could have done more for the headed goal, but other than that he barely put a foot wrong.