Brazilian starlet Piazon was in line to start the fourth-round tie at Stamford Bridge, which takes place amid the backdrop of the Mark Clattenburg affair.
Chelsea boss Roberto Di Matteo is set to make several changes for Wednesday night's game, including possibly resting John Obi Mikel and Juan Mata.
That may occur regardless of the allegations referee Clattenburg used "inappropriate language" towards them during Sunday's explosive Barclays Premier League game against United.
Clattenburg, who faces police and Football Association investigations, is understood to categorically deny any wrongdoing.
Mikel, Mata, Oscar and Eden Hazard could all be given the night off, making way for Oriol Romeu, Victor Moses, Marko Marin and Piazon.
Eighteen-year-old Piazon, who has recovered from a hamstring strain, made his first-team debut in last month's 6-0 third-round thrashing of Wolves.
He told Chelsea TV: "I was just a bit upset after the game because I didn't score but I think it was good, my first official game for Chelsea, so I was happy anyway.
"I was feeling okay and the level was a bit harder, of course. The players are bigger, stronger.
"This part, I found a bit difficult, but it wasn't a big problem.
"I think playing a better level will make you think in a better way."
Clattenburg will be replaced in the middle of what could be a febrile atmosphere at Stamford Bridge on Wednesday night by Lee Mason, who United boss Sir Alex Ferguson jokingly wished "Good luck!" even before the serious allegations about Clattenburg emerged.
That was because of the decision by Sunday's referee to send off two Chelsea players and failure to spot Javier Hernandez's winning goal should have been disallowed for offside.
Fernando Torres and Branislav Ivanovic are both suspended on Wednesday night following their red cards on Sunday, with John Terry also banned.
Ashley Cole was likely to be among those rested and is suffering with an ankle problem anyway, while Frank Lampard remains out.
Daniel Sturridge and Cesar Azpilicueta therefore look certain to start.
Meanwhile, Chelsea last night refused to confirm or deny reports they had beefed up security for this evening's game in the wake of the crowd trouble that also marred Sunday's clash and led to a steward being hospitalised.
A club spokesman said: "We keep security under continual review."
It is understood the Blues have taken the unusual step of including a warning to fans towards the front of their matchday programme for Wednesday night's game.
The message is thought to be a reminder that supporters have a responsibility for helping ensure the safety of everyone in the ground.