The mercurial Frenchman was strongly dispossessed just inside his own half by Crewe captain Luke Murphy, who duly sent the ball forward and watched Max Clayton convert from close range to put the Alex into a two-goal lead just four minutes into the second half.
It proved a decisive blow to Southend's hopes of getting back into the game after Murphy's impressive early opener, but midfielder Mohsni, who put in a below-par performance at Wembley, felt the goal should not have been given.
"Of course it's a foul because he pushed me from my back," he said.
"I did it in the first half and he (the referee) gave a free-kick so he has to give me the free-kick.
"When you lose the ball in this position Crewe are a good team. It's really, really a shame but we have to move on.
"In the league we have five more games to play, maybe come back here and have a chance to win in the play-offs. We will see."