Longstaff picked up an ankle injury in the 2-0 loss at Bournemouth before the international break and he subsequently missed last weekend's 4-1 home win over Chelsea in the Premier League.
Willock, meanwhile, was also absent for the match against the Blues after recently aggravating an Achilles problem sustained when recovering from his hamstring injury over the summer.
While Willock is considered a major doubt for the clash with PSG, Longstaff will not be involved and remains in the club's packed treatment room along with Sven Botman (knee), Callum Wilson, Matt Targett (both hamstring), Dan Burn, Elliot Anderson (both back), Harvey Barnes (foot) and Jacob Murphy (shoulder).
"We are pretty much as we were at the weekend, but just with a few absentees with the lads who can't play in the Champions League," Howe said at a press conference on Monday evening.
"Joe [Willock] is seeing a specialist tomorrow, we are still unsure on him. Sean [Longstaff] has not travelled."
Despite the large number of absentees, Howe has talked up the positive atmosphere within the first-team squad, especially after the "pivotal" victory over Chelsea on Saturday.
"The mood of the group is very good, especially after the result at the weekend," said Howe. "That was a pivotal result for us as it's very easy to feel sorry for yourselves or to focus on the players who aren't with us.
"The lads did incredibly well to blank that out and to just perform - and perform at a very high level to beat a very good Chelsea team."
One of the standout performers against Chelsea was 17-year-old Lewis Miley, who provided an assist for the opening goal scored by Alexander Isak on his home debut in the Premier League.
Howe has confirmed that Miley is in contention to retain his place in the starting lineup for Newcastle's crucial contest with PSG, saying: "There's every chance he [Lewis Miley] will be involved again.
"Let's see if he'll start. But what I liked about his performance against Chelsea was actually how he was after the game. He was the same Lewey as if he hadn't done anything. It was just another game for him and I think that's how he needs to approach this game.
"Sometimes the more you build up a game, the worse your performance can be so he's got a great temperament for these sort of occasions."
Newcastle claimed an impressive 4-1 victory over PSG in the reverse fixture at St James' Park in September, but they have since lost back-to-back games against Borussia Dortmund and have slipped to the bottom of Group F, two points behind BVB in second and three adrift of leaders PSG.
Howe has acknowledged that Tuesday's match is a "defining moment" in their Champions League campaign, but he is confident that his Newcastle players can control their emotions in front of a heated Parc des Princes atmosphere.
"I think it's good to have that mixture of emotions going into a match," said the Magpies boss. "We need to be a team playing on the edge as I think most teams are at their best when they have that different emotion running through them.
"I expect that from the players tomorrow and we're going to need every ounce of emotion and energy in order to get a result.
"This is the defining moment of our Champions League campaign. We're well aware of where we sit in the group and what can happen tomorrow so we're going to give it everything."
Newcastle have only won one of their last seven European away games against French opposition, but PSG have suffered defeat in six of their last eight meetings with English clubs in the Champions League.