Chelsea defender Antonio Rudiger will miss the end of the season because of a knee injury.
The 26-year-old Germany centre-back limped out of the 1-1 Premier League draw at Manchester United on Sunday.
The Blues confirmed that Rudiger had "successful" surgery on his left knee meniscus on Tuesday.
Andreas Christensen is expected to fill in for Rudiger, who arrived in a £29m deal in July 2017 and has made 44 appearances for the Blues this season.
Chelsea are in the fourth and final Champions League qualification spot, two points ahead of Arsenal.
However, the London rivals are also in separate Europa League semi-finals, with an additional Champions League spot on offer for the tournament's winner.
Chelsea go to Eintracht Frankfurt for the first leg of their European semi-final on Thursday, with matches against Watford and Leicester remaining in their league campaign.