Koeman said this morning: "I'm worried after what I saw yesterday.
"I wasn't worried after Chelsea. The reaction should have started in the second half against Tottenham.
"But that and the first half against Atalanta is enough reason to be worried."
He added: "After this press conference I'll have a meeting with the key players.
"It's time for the experienced players to stand up."
On Wayne Rooney's return to United, Koeman added: "He has nothing to prove, of course not.
"Everybody knows his quality. He's one of the players who needs to react and bring the level of the players we expect from players like Wayne."