The former Feyenoord coach still has close ties to his old club.
Koeman wrote for De Telegraaf: "I've been having fun watching Feyenoord.
"My old club played a really good European team. People whine about the many changes of Mourinho, but there were all internationals on the field. Feyenoord was tactically good and defended well.
"They made a few small chances and utilized one in an excellent way. They will see that you can get a good result against a club with much greater possibilities."