United's second-string host their Saints counterparts at St. James' Park requiring a positive result to pursue their hopes of a Division Two play-off spot.
Westendorf, who joined Newcastle's Academy in January after scoring two goals in four trial appearances at under-23 level, admitted the squad are determined to secure a much-needed victory following an untimely three consecutive league defeats.
"It's a big game," the Bremen-born forward told nufc.co.uk. "We were disappointed after Monday night's (4-2) defeat against Wolves. Our aim is to make the play-offs, which is still possible, and we are going to make sure we work as hard as we can to fight for those spots.
"We are now focussed on the Southampton game and have to win that match as well as the final game of the season.
"To play at St. James' Park is always a pleasure. It's a beautiful stadium and the first-team have been brilliant there since Eddie Howe has come in. It's always an honour to play there. I haven't scored there yet so hopefully I can get my first goal at St. James' to make it even sweeter!
"We won our last game there (against West Bromwich Albion) but we don't really like games like those, being two goals down and having to fight back. We want to start quick and fast against Southampton and finish the game as early as possible. It will be a tough one but the whole team are ready for the challenge."
Newcastle's youngsters suffered a 4-2 defeat against play-off rivals Wolves at Molineux on Monday, with Westendorf registering his sixth goal of the season at under-23 level.
The 20-year-old, who spent last season at Isthmian League North Division side Barking FC, is hopeful the under-23 squad can take inspiration from the first-team's performances in recent weeks.
"Wolves are a good team, sitting in second place of the table, but I felt we had our chances," he added. "We made mistakes but I thought it was a tight game but we let ourselves at the end.
"That's in the past now and we need to learn from our mistakes to make sure we don't repeat them against Southampton and get three points.
"It's important we try and keep clean sheets each game. You can see how important they are with the first-team as they aren't conceding many at the moment so we're hoping we can do something similar."
Admission for Monday's Premier League 2 clash at St. James' Park is free for season ticket holders and Members, otherwise it's £3 for adults and £1 concessions.
