The top-ranked Spaniard pulled out of the best-of-five championship because of tendinitis in his knee.
"Even without Nadal, Spain is a strong team,'' Mancini said in Mar del Plata, the beachside resort town that will host the final. "We can't relax.''
The best-of-five series opens Friday with two singles matches. Doubles is Saturday, with the reverse singles Sunday.
"We're doing well. The team is good, calm and very motivated,'' Mancini said. "All the points are winnable. We have a very professional team that can go after every one of the points.''
Juan Martin del Potro and David Nalbandian will play singles, but the doubles team will depend on the "results and intensity and length of the matches,'' Mancini said. Nalbandian, who is 16-4 in Davis Cup singles, could join Jose Acasuso or Agustin Calleri for doubles.
Nalbandian, ranked No. 11, is sorry Nadal won't be playing.
"He's a friend and missing a Davis final is never nice,'' Nalbandian said. "It's unfortunate that he didn't come. Without Rafa things change slightly, but we're staying focused on what we have to do to win the series. It doesn't matter who we're playing.''
Spain is going with David Ferrrer, Feliciano Lopez, Fernando Verdasco and Marcel Granollers. The Spanish team has complained that the hard surface was slightly irregular and the ball didn't bounce well in spots.
"The court is very good,'' Del Potro said. "The bounce is normal for this type of surface and I didn't sense anything weird.''
Del Potro, who is expected to play two singles matches, began training with the team Monday after returning Saturday from the Masters Cup in China.
The 20-year-old Argentine is ranked No. 9 after beginning the season No. 50. But four straight victories heading into the U.S. Open pushed him well up the rankings.
"Now I have to put all my effort into the Davis,'' Del Potro said. "It's the most important objective for Argentine tennis in the last few years.''
Argentina has twice reached the Davis Cup final, but lost both times, in 1981 and 2006.
"The season was good,'' Del Potro said. "But I hope to finish it winning the Davis.''
Del Potro knocks Nalbandian out at Madrid
Mancini Wins Serie A Coach Of The Year Award
