The man was taken to hospital after falling around five metres from section 126 behind the goal into the moat towards the end of the first half at the Amsterdam ArenA.
Ajax tweeted on Tuesday evening: "After 39 minutes in block 126 (south 1st ring) from row 1 a supporter fell in the moat. He was badly injured and taken to hospital."
Amsterdam Police added on Twitter: "Man from the first ring fallen into £arena. Injury still unknown, the victim is transported to hospital."
That was soon updated with: "Man who fell from first ring £arena (approx 5m) during 1st half £ajabar was transported to hospital with serious head injuries."
Pictures soon surfaced of the fall on Twitter along with rumours that the supporter had died - talk of which was swiftly quashed by the Amsterdam Police with a post of: "No update yet available on victim ££arena during ajabar. Rumours that the victim would be deceased are not correct!"
Former Ajax striker Klaas Jan Huntelaar, now with Schalke, was quick to respond on Twitter, posting "Ban the moats" soon after the incident.