The defending champion's withdrawal comes as a blow to the tournament.
Day absorbed a left hook which sent him falling backwards and his head bouncing off the canvas.
Rampant Reds will savour facing United without 2 of their best players.
Latest injury for Brazil superstar will keep him out for a while.
Dutch goalkeeper was released in the summer but could not find a club.
PSG forward felt a muscle injury and was taken off as a precaution.
Real Madrid say Kroos has injured his left thigh muscle but did not say for how long he would be out.
Ole Gunnar Solskjaer not happy with artificial surface at Kyocera Stadium.
A PSG side missing an injured Neymar were dumped out in the last 16 during a game with Real Madrid.
PSG will also be without Neymar as he has to serve a three-match suspension in the Champions League for insulting a referee.
Vincent Kompany backs John Stones to prove doubters wrong soon.
Farid Kamil says he lost consciousness after smoking a cigarette.
Costa hurt his left thigh in the final minute of the team's 2-1 friendly win over Juventus last Saturday.
Serena is hoping for a 24th career Grand Slam title at the US Open, but it is unclear if injuries will continue to set her back.
The Brazilian goalkeeper sustained an injury and needed support to limp off the field during Liverpool's winning match over Norwich.
Grand Slam champ Maria Sharapova has her hopes dashed over a comeback after being defeated by Anett Kontaveit at the fourth-round of the WTA Toronto tournament.
Nathaniel Clyne sustained the injury during last week's 3-2 defeat by Borussia Dortmund and has flown home for treatment.
With reigning champions Australia due to play England in the semi-finals, Matthew Wade and Mitch Marsh have been added to the squad.
Vikay Shankar has managed scores of 15 not out, 29 and 14 in his three matches in the World Cup.
A second scan shows Jason Roy's injury is much improved, boosting hopes he will be fit to play in the match against India at Edgbaston on Sunday.
Juan Martin del Potro will miss next month's Wimbledon after scans revealed he had re-fractured his right patella.
The defeat compounds a disappointing grass season so far for Alexander Zverev, who suffered another early exit on home soil in Stuttgart last week.
Since winning the 2009 US Open, Juan Martin Del Potro has been robbed of large chunks of his career, undergoing various surgeries due to injuries.
Playing through pain requires a change in mindset and Nadal says he was very close to calling it quits after Indian Wells.