Zsolt Poloskei has been ruled out for the season at Liverpool due to a knee injury.
The midfielder is on-loan from MTK Hungaria and has only just comeback from long-term injury.
"We feared the worst for Zsolt and unfortunately scans have revealed that he has damage on his knee and he needs an operation," McMahon told liverpoolfc.tv.
"It's a major setback for the lad and for the club. We are just devastated for Zsolt because he has shown some fantastic things in his play. He is a good footballer with a fantastic attitude.
"The season is finished for Zsolt but he has come back from a bad injury once and hopefully he can do that again."