Lallana has started 25 matches, Henderson's started 42.
I'm loathe to get into a stat-debate as it serves very little purpose, but it's worthwhile ensuring we're comparing apples to apples Dreamy. Up until the end of February, Lallana had the best minute-per-goal and minute-per-assist ratio of anyone at the club. He was in the top 15 in the league for it too if memory serves me correctly. So, contrary to your judgement, he does "score and create goals" when he plays. Since then he's been injured and out of the side.
His minutes per goal ratio even now is still better than Henderson's, not that it matters to me but you brought him into this debate in some inglorious attempt at making a point, so I thought I'd set you straight.
I can't help with what you "expected" though unfortunately.
Where to begin.
Lallana was bought to score and create goals. Comparing his sh*te stats in this department to the rest of our teams sh*te stats hardly proves he's been a success. It just proves he's been less sh*te than other players (bar, Coutinho, Sterling & Henderson)
He was our big summer signing. With that came expectation that he would score and create a lot of goals. The fact he had already done it at Southampton lead most people to expect him to hit the ground running. Injuries at the start of the season aside he simply hasn't produced anything near enough for his transfer to be seen as a success.
I compared him to Henderson to show you that even though Henderson is seen as hard running, energetic box to box midfielder (yes they do still exist) Lallana still doesn't exactly blow him out of the water. Lallana's goal and assist return should blow Henderson's away. Even taking the extra games away, Henderson smashes Lallana on the assists stat and is comparable on the goal stat. Now considering Lallana was never a high scoring midfielder to begin with you would at least expect him to have a high assist rate. He doesn't and when that's what you are bought for and don't deliver of course people will be disappointed.
Swansea at home aside I struggle to think of games where Lallana was the stand out player in our team. Again at 26m you would expect him to be the stand out more often that not in 30 odd games.
You had a right go at Coutinho for not producing important goals in big games, well where is the finger pointing at Lallana? He's produced f&ck all in the big games when we needed him most.
He's a very capable footballer, we can all see that, but this season has been a big disappointment and he is definitely one of the reasons why along with some other poorer transfers this season has been sh&te.