Wales international Allen followed Brendan Rodgers from the Liberty Stadium to Merseyside during the off-season for a fee believed to have been around the £15million mark.
There were plenty who questioned whether Allen was worth a fee of that size, despite his fine performances for Swansea last season and during the Olympics in the summer.
The 22-year-old has quickly proved his doubters wrong, settling quickly into the Reds midfield with his assuredness in possession perfectly fitting Rodgers' gameplan.
Having come through the youth system at Swansea, the last-16 clash at Anfield will doubtless be a strange experience for Allen, but Monk says the Swans squad have been delighted to see their old team-mate make such an impression.
The defender said: "He is a top player. Everyone knows his quality as a player, and he is a great lad too.
"A few of us have had a bit of banter with him, winding each other up.
"But it will be great to see him, I am really glad to see he is doing so well.
"He is a great lad and a great talent and he is proving it now at a very high level."
While Allen can expect to get a reasonable reception from the Swansea fans, Rodgers is likely to face some hostility from those disappointed by his decision to leave south Wales, despite the huge career opportunity a move to Liverpool represented.
Monk was Swansea club captain for Rodgers' two seasons at the helm, and he hopes the fans will remember the service the Northern Irishman did the club in getting the Swans over the final hurdle into the Premier League.
"When you look at it, it is the hardest hurdle to make it from the Championship to the Premier League, and then to sustain it," he added.
"We managed to sustain it convincingly during the first season and that has given us the platform to push on into the second season.
"It is important the fans remember that but we are there to win a game, all that matters is what we do as Swansea City.
"I think if a few want to show their views that is up to them but the main thing I would say to the fans is back the team from start to finish, whatever you do before or after game is entirely up to them, but during that 90 minutes just focus on the boys on the field as that is all that matters."