If Ole cant gives Lingard regulate football, then staying at the club obviously isn't the best for Lingard's career.As long as looking after his career means getting him a good move away thats fine, not if it means holding onto a substandard player on a wage that could be much better spent elsewhere purely because ole has known him a long time and likes him.
Also, players have contracts. Lingard can legitimately stay for another 12 months and then become a free agent.