The midfielder started centrally alongside Gylfi Sigurdsson in Tim Sherwood’s 4-4-2 system and was a focal point as we dominated the first half.
Goalless at half-time, Emmanuel Adebayor crashed home a volley to open the scoring on 67 minutes and it looked like a place in the semi-finals was on.
However, West Ham levelled through Matt Jarvis with 10 minutes remaining and five minutes later, Modibo Maiga headed home the winner.
Mousa came closest to grabbing a late leveller in added time but goalkeeper Adrian produced his best save of the night to turn away his drive from 20 yards.
“We started well, we were very motivated, aggressive and you see how much we wanted to win,” said Mousa. “The fans were behind us from minute one, that was important. It’s difficult because I thought we were going to win this game, in the end we lost, so we’re all disappointed.
“We didn’t win but if you see how we played, the passion, that motivates us as well. We need to keep going.”