Manchester United are in need of extra defensive reinforcements at present and a new face has been spotted in training. Lisandro Martinez is not the only Manchester United defensive injury. Luke Shaw, Aaron Wan-Bissaka and Tyrell Malacia missed the training session yesterday, with only Shaw having a chance to recover for the weekend against Luton. Victor Lindelof might be moved out to full-back as cover, creating space for a young centre-back to join Willy Kambwala in first team training. Enter Louis Jackson, a highly rated centre-back prospect who has been part of the under-18s side which has…