#4: First-half injuries disrupt Alan Pardew's plans
Going into the second-half with a 3-1 lead, one would expect any manager - let alone Alan Pardew - to ask his team to defend as a unit, prioritising a solid defensive display as opposed to going forward more.
However, due to the lack of reinforcements, Pardew was forced to hold off on any positional changes he perhaps would have otherwise made.
Kieran Gibbs, signed in the summer from Arsenal, sustained a hamstring injury and was unable to continue any further. Ahmed Hegazy came on in his place, being asked to play in a relatively unnatural position at left-back - unsurprisingly coming second best in his duel with compatriot Salah down the flank.
It went from bad to worse for Pardew after Hal Robson-Kanu was also forced off minutes later too with a similar injury problem. Matt Phillips replaced the Welshman, playing as their number nine - whereas he prefers playing out on the wings.
He was virtually forced into the third and final alteration in the second-half, as Gareth Barry was looking increasingly fatigued and no longer providing the Baggies with the defensive solidity he usually does. Claudio Yacob came on in his place, as their third and final substitution. Come the latter stages, both Jake Livermore and Evans also appeared to carry knocks but managed to labour on, pushing forward until the final whistle.
Had Gibbs and Robson-Kanu not have succumbed to injury, West Brom probably would have seen the game out more comfortably than they did in truth. Liverpool threatened several times during the latter stages with time running out, but the visitors edged through and booked a place in the fifth round of this year's competition.