Lucas has to start, they are likely to play on the counter and he'll be key to stopping them. The last thing we want is Gerrard having to chase counter attacks. Lucas can prevent the ball into Benteke, Gerrard at DM can't do that.
Also, if Gerrard starts at Wembley, Rodgers doesn't have the nuts to bring him off if its going wrong, or if he's getting tired. Best to start him on the bench and bring him on for 30 mins.