The Belgium international was replaced by Willy Caballero, and manager Antonio Conte said Courtois had a "muscular injury," per BBC Sport (h/t Alex Young at the Evening Standard).
The 25-year-old picked up the injury in training but would not have featured in the FA Cup clash even if he had been fit, according to journalist Kristof Terreur:
Caballero is Courtois' back-up at Chelsea and has featured in every round of the FA Cup so far. The Argentinian has also conceded just one goal in four games, in their third-round replay against Norwich City.
However, Conte has said that he does not have a specific FA Cup goalkeeper and that he would have to choose between the two, as shown by Goal:
Courtois has come under scrutiny after his performance in the 3-0 UEFA Champions League defeat to Barcelona on Wednesday. Two of Lionel Messi's goals went through the goalkeeper's legs as the Blues were knocked out of the tournament at the last-16 stage.
ESPN FC's Liam Twomey said the 'keeper was to blame:
The Chelsea stopper spoke about the goals after the match and said Messi put the ball at his "weakest point," as shown by Football on BT Sport:
Vaishali Bhardwaj at the Evening Standard showed how Courtois was aiming to make amends:
Courtois is widely considered to be one of the best goalkeepers in Europe, although he has not been consistently at his best this season. There has been plenty of speculation over his future, but he has said he is committed to Chelsea, per Indy Football:
Indy Football @IndyFootball
Thibaut Courtois: “I’m committed to Chelsea. I have a contract until next year and I will be here. I will do my best for it. Obviously there is no extension signed but I don't think now is the moment to do it. I think it is better to wait until the end of the season." https://t.co/i0MO31qmAs
The goalkeeper must now wait to make amends for his performance against Barcelona as the Premier League pauses for an international break. The Blues face a crucial game against Tottenham Hotspur next up in the top flight on April 1 and will be hoping Courtois is fit and available.