Early on in the match at Upton Park Liverpool were by far the brighter and more inventive of the two sides, with Glen Johnson enjoying plenty of action in the final third in particular as he got forward with regularity to cause problems for his former side.
Less than half an hour into the game though the Reds were forced into their first substitution as Jose Enrique was replaced by Joe Cole. Stewart Downing shifted to left-back with Cole moving into the front three, and the game was changed completely into West Ham's favour thereafter.
It wasn't that Downing or Jose Enrique had had a magnificent impact on Liverpool's play on the ball, nor that Cole was anything below what Reds fans have come to expect from him, but instead a reduction in the space Johnson was able to get in the final third.
Previously Downing had hugged the touchline on the right, leaving a lot of space infield of him for Johnson to break into towards the penalty area and occupying the West Ham left-back in process.
With Cole (and Raheem Sterling, as the two switched positions) more inclined to break infield Glen, Johnson had less room and thus less opportunities to make up the numbers in the final third, instead having to hold his position wide in case of any counter attacks.
West Ham went on to be the stronger side for the best part of half an hour after that—but suffered a similar fate around the 70-minute mark when Mo Diame went down injured.
Liverpool had made a change just minutes beforehand, Lucas replaced by Jordan Henderson, and the switch in midfield energy suddenly swung the attacking emphasis back in Liverpool's favour as they out-played West Ham through the middle very quickly.
The movement was better and two very simple yet effective runs into the penalty area, of the kind the forwards should have been making all along, resulted in two goals for Liverpool.
It's also worth noting, of course, that Liverpool's second half goals were scored and made by substitutes.