Manchester United fans are baffled at how Casemiro looked less composed in the heart of midfield than teenage sensation Kobbie Mainoo after his performance against Luton Town. Erik ten Hag started Casemiro alongside Kobbie Mainoo, who has had a breakthrough season at Old Trafford, for the Premier League clash at Kenilworth Road on Sunday. Casemiro was widely hailed for his impact at Old Trafford during his debut season at Manchester United, but the former Real Madrid man has faced criticism this season. The 31-year-old Brazilian midfielder has had to contend with injuries in the current campai…