The Dutchman expressed his alarm at the manner in which his side capitulated in the first half against Atalanta on Thursday night, falling 3-0 behind before the interval as they began their Europa League group phase campaign in awful fashion.
That loss in Italy came on the heels of a 2-0 reverse to Chelsea and a 3-0 home defeat to Tottenham and the reaction that Koeman was expecting to see, particularly from those players he regards as leaders on the pitch, against Atalanta didn't materialise.
He hinted that he won't be going for big changes to his line-up for tomorrow's game against Manchester United at Old Trafford, nor will he be turning to youth like Jonjoe Kenny or Tom Davies; rather he will be looking to those older pros to start delivering.
“I will have a meeting with most of the players, with the key players," Koeman told the media. "Not a young talent of 18 or 19 because it's time for the experienced players to stand up.
"I am worried about what I saw yesterday. I wasn't so worried after Chelsea. It starts with the reaction in the second half against Tottenham. That 45 minutes and the first 45 minutes of yesterday are enough reason to be worried.”
“I hear a lot of times the name of Kenny," Koeman continued with regard to the growing clamour around blooding the England U-21 international as Cuco Martina struggles at right back in Seamus Coleman's absence.
“I [have said before that] the under-23s competition is not the Premier League and the standard of level of performing is a training session by the first-team.
“What I mention before. He is on my list, of course, but I think it's not the moment to put Kenny in. If I don't get enough on Sunday and next week then maybe yes there comes an opportunity for the boy, because he is a talent.
“But you can't mention if someone is performing well for the U23 he is ready for the first team, it's too easy, it's too easy.
“That is for Kenny, that is for... Tom Davies was an exception, an exception. But Davies is now a little bit more struggling."