Just said this in his presser
"We have a lot of doubts. Last season we had a good defensive organisation but we have lost our strengths - at the moment," Koeman said.
"If you lose that against the big teams in the Premier League, and yesterday too, it was painful because they showed more ambition, aggression, and defended so tight.
"And the space we gave to their (Atalanta) strikers was different. I don't tell the media what I tell the players.
"After this press conference I will have a meeting with most of the players and the key players. I think it's time for the experienced players to stand up.
"Defensively there are too many mistakes. I am worried what I saw yesterday. I wasn't so worried after Chelsea. But it started second 45 minutes against Tottenham, and the first 45 minutes yesterday.
"We have some players with doubts, and I don't want to be too individual, because it's always a team performance, but we need to be more aggressive."