you're probably right abou that-it is not worth it to sell Gerrard. I don't think selling Gerrard to get another player is worth it at the moment, no matter who the player is.
Aimar wouldn't give them the wide option that Kewell does though and imo Liverpool don't really need another midfielder right now. They've got Alonso, Gerrard, Hamann, Garcia and Kewell, plus Riise can play there. They should concentrate on getting a striker for now and then depending on how much money is available at the end of the season a midfielder.
The key right now for Liverpool is keeping Stevie G which means qualifying for the Champions' League.