United, so often all but unbeatable on their home patch, have already lost four home league games this season, the latest coming in Wednesday's defeat against Tottenham.
It has ensured David Moyes has not enjoyed as smooth a transition as he would have liked after taking over from Sir Alex Ferguson.
But while De Guzman believes United are not as strong as last season, he knows a trip to Old Trafford is no easy assignment.
"They're not at the same level as they were last season, not that lethal, but I think they've been doing pretty well in the last couple of weeks," said De Guzman.
"They've been scoring goals and getting points home and away.
"You can't underestimate Manchester United, they have a very good squad, but I think we did well against them last season away, we were just unlucky in the end.
"We're going there to get a result. We take hope from the fact they have lost some games at home this season and it's the FA Cup - it will be an open game."
Swansea enjoyed knockout success in the Capital One Cup last season, but were knocked out of that competition after just one game.
Now, with Swansea having failed to win in their last six league games, De Guzman hopes the FA Cup can provide a timely boost.
"It's another prize and we all take it seriously," he said.
"The FA Cup is a great cup to win. We did very well last season in the Capital One Cup and unfortunately we're out of that.
"The FA Cup is another chance to win a trophy. It's very important for us.
"It is a distraction but I think it's a little breather for us from the Premier League."
Swansea face United after a 3-2 defeat to Manchester City on New Year's Day.
De Guzman believes there was plenty of room for encouragement for the Swans, but believes errors are costing the Welsh club.
"I think we did very well in the first half," said De Guzman. "But we've got to stop letting teams score the first goal. It's very important for us to get the first goal.
"I think we did a lot of positive things, we had a lot of shots on goal and created more chances. That's very important for us.
"(Wilfried) Bony did very well up front for us, holding the ball up at times and keeping the team composed."