He believes several of his players, most notably £24m Klaassen, are struggling to get to grips with the Premier League.
Koeman said: “Klaassen has not had an easy start. [But] There is absolutely no doubt about his qualities.
“There are so many examples of players who were the best in their country, who were ready for a big transfer, and still found it tough here.
“I know bigger stars than Klaassen who needed an entire season before they could deliver.
“I read and heard all the critics in his first season. It was unbelievable what he had to endure. However, in the beginning, he was struggling to survive. When you look at him now he is performing at the top level, and so important for Man United week after week.''