Having kept the hosts out during the first half, the visitors conceded the only goal of the game three minutes after the restart when Demba Ba got around the back of the United defence to volley beyond David de Gea.
Ba scored a similar goal against the Red Devils while playing for Newcastle United and with that in mind, Ferguson has conceded that his players should have dealt with the striker's threat more effectively.
"We spoke about Demba Ba, with the players' experience of playing against him when he played for Newcastle. He scored that exact type of goal against us a while back and we were caught napping," Ferguson told ITV Sport.
The loss means that United's wait to win the FA Cup will extend by at least another year. They last lifted the trophy back in 2004.