Manchester City have had a number of injury problems already this campaign and Pep Guardiola believes that is down to the demands being placed on his side.
Iker Casillas called curtains on a glorious career which included a World Cup trophy and two European Championships.
Democratic Republic of Congo international Chancel Mbemba scored twice as FC Porto defeated old rivals Benfica 2-1 on Saturday to win the Portuguese Cup for the 17th time.
Iker Casillas played 167 times for Spain, also winning two European Championships in 2008 and 2012, and made more than 700 appearances for Real Madrid.
Twitter broke out in concern as news of Iker Casillas's heart attack spread.
Liverpool crushed FC Porto 4-1 in Portugal on Wednesday to win their Champions League quarter-final tie 6-1 on aggregate.
Liverpool's only regret may be not building a more emphatic lead to take to Portugal for next week's second leg.
Liverpool were drawn to play FC Porto in the knockout rounds for the second year in succession.
VAR was initially set to be rolled out in the Champions League from the start of next season. However, it will now make its debut six months earlier than anticipated.
United are aiming to close the gap on Manchester City, who won the Premier League by a record 19 points from their local rivals.