The Dutchman netted a last-gasp equaliser on the same ground when the sides met earlier this season in the FA Cup third round.
At the time, van Persie was enjoying a rich vein of form in front of goal. Indeed, that dramatic cup strike was his ninth in nine games. Now the 29-year-old travels to East London having netted just once since late February.
Mind you, that goal came in the Reds' last outing – away against Stoke City – to end an 11-game goal drought and, judging by Robin's celebrations, went some way to restoring the Dutchman's confidence.
Sir Alex now believes the striker can prove pivotal as the season draws towards its climax.
"There are always going to be question marks from the press – and particularly from the player himself – when a striker hasn't scored for a few weeks," he told MUTV.
"I know what strikers are like: when they are scoring they think they are going to score all the time, and when they’re not scoring they wonder when the next goal is going to come along.
"So it was a great goal for Robin [to score against Stoke] in that respect, and hopefully he kicks on from there."