The 2009 winners beat Almeria 3-1 at home in La Liga on Saturday before making the journey to Ukraine and Guardiola told a news conference in Donetsk he and his staff would assess how the players were feeling before the game at the Donbass Arena.
"Rest what? We are playing for a place in the semi-finals against a very good team and we have to start very strongly," said the former Spain and Barca midfielder.
"There are 90 minutes still to play. The idea is to go out there to win the game and try to score one or two goals.
"That's the idea we have always had and the one we'll maintain. Tomorrow we'll see how people are feeling physically."
If they protect their comfortable lead, Barca will likely play Spanish rivals Real Madrid home and away in the semi-finals as long as Real avoid any dramas at Tottenham Hotspur on Wednesday after winning the home leg 4-0.
Before that likely last-four matchup, Real host Barca on Saturday in La Liga and the two giants clash again in the final of the King's Cup on April 20.
"We are making a mistake if we play with the Real Madrid games on our minds," Guardiola said.
"Each game is a rehearsal for the next but this one is important enough not to think about anything else."
Shakhtar coach Mircea Lucescu was not holding out much hope of causing an upset on Tuesday.
"It is clear there is very little chance for us to progress to the next stage," he said at a news conference. "But we would like to go out with our heads held high."
