Ralph Hasenhüttl agreed that the red card shown to James Ward-Prowse was the pivotal moment in the 3-1 defeat to Chelsea, coming at a point when he felt his side could have gone on to win the game.
The Saints captain was shown a straight red in the 77th minute by referee Martin Atkinson, after he reviewed his initial decision of a yellow, following a foul on Jorginho.
Hasenhüttl had no complaints with the decision, but it was an unfortunate one nonetheless, with Ward-Prowse, who had levelled from the penalty spot not long before, sensing an opportunity for what would have been a crucial interception just outside the Chelsea box, only to end up catching the Chelsea midfielder's ankle late.
Saints had been looking threatening after levelling the game at 1-1, but the shift caused by that dismissal put them on the back foot, with late goals from Timo Werner and Ben Chilwell ultimately winning it for Chelsea.
"We really tried everything what we can do to make it as tough as possible for them, and I think we did that for a long time," said Hasenhüttl, who is now faced with the prospect of losing Ward-Prowse for the next three matches.
"It was just at the moment where you could feel that, ok, now we have more to give, we can maybe hurt them really and take three points, then the red card came and at that moment you know it is getting tougher, because with 11 it's difficult to defend and with ten even more.
"He knows that he has made a mistake there I think, but this is our way of playing sometimes, is we are trying to win balls high up the pitch, but this is a pity because he is out now for us and he is an important player for us."