Atletico travel to the Argentine city of Rosario for the first leg of the semi-final after a month's break in the Libertadores and the Brazilian championship while the 2014 World Cup hosts staged and won the Confederations Cup.
"We've been missing the ball," Ronaldinho, overlooked by Brazil despite his fine form for Atletico, told the club's TV channel.
"This break has come at a good time. We're all going to come back keener to play and reach the final. It's going to be another 'final', the hardest match we've had so far," he said of the game at the Marcelo Bielsa stadium.
Jo, joint top scorer with team-mate Diego Tardelli on six goals, and Bernard should be confident after their role in Brazil's success.
But centre-back Rever is out of both legs of the semi-final after being sent off for dissent at the end of Atletico's dramatic quarter-final win over Tijuana.
With the score 1-1 and 3-3 on aggregate, the Mexican debutants looked set for a shock victory in the second leg in Belo Horizonte. But goalkeeper Victor saved a penalty with his foot to keep Atletico in the tournament on the away goals rule.
Ronaldinho's side are looking to win South America's top club title for the first time. Victory over Newell's would put them into a final against three times champions Olimpia of Paraguay or Independiente Santa Fe of Colombia.
Olimpia, Paraguay's most successful club who last lifted the South American crown in 2002, are at home in the first leg at the Defensores del Chaco stadium in Asuncion on Tuesday against Colombia's Santa Fe.
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