Martin Dowden, BBC Scotland at De Kuip
The timing of the opener was such a hammer-blow for Celtic. To that point they had more than matched their opponents, who struggled to engineer consistent pressure.
Celtic had a good balance of being fairly solid, with good shape, composed on the ball in the main and carried some threat. They had chances and one or two better decisions may have paid dividends.
The opening goal was absolutely crucial. Feyenoord returned with more purpose and tempo after half-time. It counted and Celtic's night utterly fell apart.
With nine men on the pitch it was mission impossible. So it proved.
They will take some heart from that opening half but major lessons will need to be learned ahead of the visit of Lazio.