Unless Sturridge gets crocked for a long time, Lambert won't play the minutes he got at Southampton (about 2,800 to 3,100+). If we assume he gets about 10 full games in the league and comes off the bench at half time for the remaining 28 games, that's still about 24 league games in all (2,000+ minutes). Based on his goals to games/minutes numbers at Southampton, he hit around a goal every 2.5 games.
8 goals from 24 full league games' worth of minutes is actually just a couple of goals off that rate. It really isn't a big knock on him. Besides, he won't be taking the penalties with Gerrard in the team, and with a less consistent run of playing time, that drop shouldn't come as a surprise. Like I said, it isn't a knock once you factor in the likelihood of the cut in his playing time.