The striker publicly celebrated the 2-1 win by posting a picture and a note to fans on his website, while match-winner Kuyt enjoyed scoring against rivals in front of the Kop again
Park Ji Sung levelled the scores after Daniel Agger's opener at Anfield, and it looked like the tie would be forced to a replay, until Dirk Kuyt's late winner sent the crowd into ecstatics.
The 25-year-old is currently serving an eight-match ban for racially abusing Red Devils' left-back Patrice Evra, who captained Sir Alex Ferguson's men on Saturday.
The Frenchman received a torrent of boos from the Merseyside fans during the tie, with one case of a racial gesture being investigated by police.
After the match, the Uruguayan posted a picture of himself celebrating the victory on his official website, with a note praising his team-mates but also the 'spectacular' support.
The note reads: "Very happy and proud of my teammates. For giving this joy to the fans who are spectacular. We passed the round in a very complicated playoff ... I'll be back soon! YNWA"
Kuyt, the winning goalscorer, posted his reaction to the fourth round triumph on Twitter, saying thank you for the support during the 'massive' win.
He tweeted: "Thanks for all the support today!! It was a massive win an great to score in front of the kop against man. united again!!"