Lascelles was taken off at half-time with a knee problem in United's comeback victory against Everton, whilst Schär will serve a two-match suspension after the Swiss defender picked up his tenth yellow card of the season against the Toffees.
Although the defensive pair will miss the clash at the Vitality Stadium, Ciaran Clark has recovered from his knee injury sustained in Newcastle's FA Cup win over Blackburn back in January.
“Longstaff and Elliot are unavailable," Benítez said. "Lascelles is feeling his knee so we won't take any risks – we’ll assess him in two or three days to see how he reacts. But Clark is available so we have lost one, but we have another one available.
"Fernández has more chances now. He was our best defender at the beginning of the season. After that he was injured and the team was doing well, so he had to wait. But he has been training well and hopefully he will be ready.”
Following an excellent victory over Everton last weekend - United's fifth consecutive victory at St. James' Park - Benítez understands the importance of another positive result.
The Magpies are currently three points adrift of the Cherries, who sit in 12th place, but only maintain a six-point gap over Cardiff City, occupants of the final relegation spot in 18th.
Benítez added: "It's a game that can change a lot of things. If we can get three points, I think we can keep pushing with more confidence without any fear that if you have a couple of bad results, everything will be more difficult. If we do that, we can do much better and, if not, we'll have to keep fighting like we have done already.
“The commitment was there and the belief was there. After making two mistakes in the first half, the reaction was very good. You can see that the confidence is high, but we need to be careful as the job is still not done. We need to keep working hard and approach every game as a final and if we do that, we can compete against anyone.
"Every game has been normally quite close and everyone was thinking about that. If you score one goal, you can get back into the game and this one (against Everton) was the perfect example. The team were trying and as soon as we scored the first goal, everyone has the feeling that we could change the game, which we did."
Newcastle defeated Bournemouth 2-1 in their reverse fixture back in November, with Salomón Rondon's brace securing a valuable three points at St. James' Park.
However, the Magpies are without a league win away from Tyneside on their last six occasions, with Benítez hopeful of rediscovering the form which saw his side pick up victories at Burnley and Huddersfield Town.
“It's a good team," the Spaniard added. "They play well, they are attacking and they score goals, but at the same time they are conceding goals. They need to be more consistent and I’m sure Eddie (Howe) is thinking about that.
"Each game is different. I remember, at the beginning of the season, our home form was not the best but we were good in away games. Now it is the opposite so hopefully we can be consistent and we can do well, starting with this game.
"I knew that sooner rather than later that we would start to win at home because we were very close. I had confidence and I think they will have that too because they are a good team who have good players that can create chances in every game."
