MANCHESTER, England — Alejandro Garnacho scored Manchester United’s second goal, then perched on an advertising board and beckoned Rasmus Hojlund and Kobbie Mainoo to come and sit alongside him.

On Sunday evening, the picture was jointly posted on Mainoo, Hojlund and Garnacho’s Instagram profiles, where it has received more than 1.5 million likes in less than 24 hours.

Hojlund has now scored just one goal in his past 25 United appearances in all competitions, while Garnacho has one from his past 28. Ruben Amorim will hope both will be able to turn their form ...