The England internationals travelled to Spain with the rest of the squad as United bid to advance on Thursday after last week's disappointing 2-2 draw at Old Trafford.
"They are available because that's why they're in the airplane, but I'll take my decisions tomorrow," Ten Hag told a news conference on Wednesday.
"Marcus had one session with the team (prior to today), but he had some more individually."
Rashford missed the first leg due to a groin injury while Shaw has been nursing a hamstring problem.
United conceded two late own goals to allow Sevilla to draw the first leg after Marcel Sabitzer's quickfire double in the first half put them in control of the tie.
"We let them in the game, I think, in the first game," midfielder Christian Eriksen said. "We had good control for the first hour.
"They've had a lot of different coaches this season, but they are now starting to win games and being in a bit safer spot. They're very direct, they're in a good shape."
Ten Hag is looking for more from his strikers.
"I think defensively we're good, with I think most clean sheets in the Premier League. Our organisation, all over the pitch, is good. In attack... we should have scored more goals during the whole season," Ten Haag said.
"But I'm confident, especially now that we have all the players on board, that we should score in every game and I'm confident we can do that tomorrow as well."
United will be without midfielder Scott McTominay and defenders Raphael Varane and Lisandro Martinez, who has been ruled out for the rest of the season due to a fractured foot.
Midfielder Bruno Fernandes is suspended after picking up a yellow card in the first game against Sevilla.
United are third in the Premier League on 59 points, three above fourth-placed Newcastle United.
