The young Magpies face the Saints' second-string at Whitley Park (kick-off 2pm BST), aiming to extend their unbeaten league run to five matches following a 2-1 away win against their Aston Villa counterparts last week.
After sustaining a thigh injury during pre-season, Westendorf made his return to United's under-21 side as a second-half substitute during a Papa John's Trophy Northern Group E clash against Lincoln City on Wednesday night and is determined to make up for lost time recovering away from the pitch.
The Bremen-born forward, who joined the Magpies in January 2022 following a successful trial, told nufc.co.uk: "Last season, I had a thigh injury and when I came back, I was training with the first-team but I got injured again. Mentally, it's tough because, when I came back, I felt quite strong and ready to go. It was a setback but injuries are part of the game.
"Now, I'm eager to get minutes under my belt and help the team on Sunday, whether I'm starting or not, to try and get the win.
"Nobody likes being injured but it does have some good sides to it as well. I worked a lot with Haydn (Rockall) and Ben Dawson and went through a lot of my video clips from last season, watching them back to see what I could improve on.
"I think you could see glimpses of that when I came on against Lincoln. Obviously, I was frustrated not to play and it was hard not being able to help the squad due to injury. Now I'm fully fit, I can't wait to get going again!"
Newcastle's youngsters, currently occupying sixth spot, will move level on points with the Saints, sitting one place above United in the Division Two table.
Westendorf, who was on target in the young Magpies' recent encounter against Southampton, a 4-2 defeat at St. James' Park back in April, believes the return of a number of squad members will benefit the squad in their bid for promotion to the top tier of the Premier League 2.
The 21-year-old, who spent last term plying his trade for Isthmian League North Division side Barking, said: "We played brilliantly against Villa last week, getting the victory and it's important that we keep going and making sure we continue the unbeaten run.
"The start of the season was tough for the team and I was injured too. We had a good pre-season and although we had a little dip in results, I don't think we are too far behind other teams and we now have a fully fit squad.
"In training and games, I think we are back to the levels we were at during pre-season and it's all about trying to get more positive results. Southampton are a strong team who beat us twice last season but that's in the past. On our day, we can beat any team we face.
"It's better having a full squad because there's always going to be strong competition. We've recently signed Amadou (Diallo) who is a good player and it's healthy to have people fighting for places. We're all performing hard in training in order to gain starts in matches but we are all very close as a group and want each other to do well."
