UEFA has banned Ajax Amsterdam goalkeeper Andre Onana for 12 months for a doping violation, just days after the club's record signing Sebastien Haller was mistakenly left off their Europa League list.

Following a drugs test on Oct. 30 last year, the substance Furosemide was found in Onana's urine. The suspension is effective immediately and applies to all footballing activities, both national and international.

- Eredivisie on ESPN+: Stream LIVE games and replays (U.S. only)

Onana has said that he mistakenly took a Lasimac tablet prescribed to his wife after feeling unwell.

UEFA has said that Onana had no intention of cheating. However, the European Football Association states that an athlete has a duty to ensure that no prohibited substances enter their body.

"We explicitly renounce performance-enhancing drugs, we obviously stand for a clean sport," Ajax managing director Edwin van der Sar said. "This is a terrible setback, for Andre himself, but certainly also for us as a club. Andre is a top goalkeeper, who has proven his worth for Ajax for years and is very popular with the fans.

"We had hoped for a conditional suspension or for a suspension much shorter than these twelve months, because it was arguably not intended to strengthen his body and thus improve his performance."

Onana and Ajax will appeal against the decision to the Court of Arbitration for Sport.

The news comes just days after Haller, a January signing from West Ham, was not included in Ajax's list for the knockout stages of the Europa League.

It is not clear whether the error was caused by Ajax, the Dutch FA or UEFA but if the club are at fault, they will not be able to amend the situation.

Coach Erik ten Hag said: "[It is] nothing to laugh about. Very disappointing, also for the player. It's a mistake in the administration by someone. We will do everything to get him on the list. In the end [club director of football] Marc Overmars and I are responsible."

Meanwhile, Ajax also confirmed exciting teenager Brian Brobbey will leave the club when his contract runs out at the end of the campaign.