Liverpool winger Ryan Babel was fined 10,000 pounds ($16,000) Monday by England's Football Association over a series of Twitter postings that criticized a Premier League referee.

Having already accepted a charge of improper conduct, Babel was also warned about his future conduct by the FA.

The Netherlands national team player posted a digitally altered picture of referee Howard Webb wearing a Manchester United jersey after being angered by the decisions in Liverpool's 1-0 loss at Old Trafford in the FA Cup on Jan. 9.

"Social network sites must be regarded as being in the public domain," Roger Burden, the chairman of the FA's regulatory commission, said in a statement. "All participants need to be aware, in the same way as if making a public statement in other forms of media, that any comments would be transmitted to a wider audience. It is their responsibility to ensure only appropriate comments are used."

Babel also wrote about Webb on Twitter: "And they call him one of the best referees. That's a joke. SMH (shaking my head)."

In Liverpool's loss, Webb ruled that defender Daniel Agger fouled United forward Dimitar Berbatov 30 seconds into the match and Ryan Giggs scored the ensuing penalty kick.

Webb also ejected Liverpool captain Steven Gerrard for a challenge on Michael Carrick. Last July, Webb was the referee in Spain's victory over the Netherlands in the World Cup final.