United were outclassed 3-1 by Liverpool at Anfield in December – a result that sealed Jose Mourinho's sacking and led to the appointment of caretaker manager Ole Gunnar Solskjaer.
Solskjaer's United have not lost in the Premier League since that trip to Merseyside, winning eight of the next nine matches ahead of Sunday's showdown.
Liverpool can move three points clear of Manchester City at the summit if they beat their rivals on Sunday.
Although coach Jurgen Klopp insists the match at Old Trafford will not prove decisive in the Premier League title race, Henderson knows his side face a huge task in trying to overcome a resurgent team with a "massive" three points at stake.
"It is a different United, definitely," said Henderson. "They are in a better moment now. They are in much better form, everyone can see that.
"They are a good team with top, top players. So it will be another tough test for us going to Old Trafford. We will need to be at the top of our game to get anything out of it, but we are in a good place as well and just need to give everything.
"Every game is the biggest game for us now. They are all massive. It would be a massive three points for us."
Liverpool and City are level on 65 points, though Henderson's side have a game in hand with Pep Guardiola's reigning champions set to face Chelsea in the EFL Cup final on Sunday.
In-form United, meanwhile, are fourth and 14 points off the pace in the Premier League.