Sir Alex Ferguson's side currently sit seven points clear at the top of the Premier League, but the Lilywhites have beaten United already this term, inflicting the Red Devils' only home defeat of the season in September.
Defoe says his side are good enough to beat the league leaders, as the north London club hunt for three points to keep them in the Champions League spots.
"What can you say about United? They are a great side with great players and it will be a big test for us," Defoe told the club's official website.
"But they know it will be difficult for them as well when they come to White Hart Lane.
SPURS V MAN UTD PREVIEW
Jermain Defoe is 8/1 with bet365 to score two or more goals against Man Utd
"Hopefully we can get the three points - and we're more than good enough to do that with the players we've got here."
Defoe has been in good form this season, scoring 14 times for Spurs in all competitions, though he is in something of a drought, hitting the net just once in his last seven outings.
The England international reserved special praise for fellow striker Robin van Persie, who has 21 goals this term for United, but says he is not the only player Spurs will have to keep tabs on this Sunday.
"[Van Persie is] on fire, scoring goals and that was always going to happen," Defoe added. "They've plenty of other dangerous players as well.
"It will be a difficult game but we're all looking forward to it."