The treble winners said the 29-year-old had signed a four-year contract after the clubs reportedly agreed an initial fee of £25 million ($32 million), with a potential £5 million in add-ons
Manchester City have agreed a £30 million ($38 million) deal with Chelsea to sign midfielder Mateo Kovacic, according to reports on Wednesday.
Ben Chilwell, Kai Havertz and Mateo Kovacic got on the scoresheet as Chelsea continued their upturn in form with a 3-1 win at Leicester City in the Premier League on Saturday.
Chelsea are looking to win their first away game in the league since October, a run of six matches which included three losses at Potter's former club Brighton & Hove Albion, Newcastle United and Fulham.
If surprise 2018 runners-up Croatia take the World Cup by storm again in Qatar, in the thick of it this time will probably be Mateo Kovacic, one of the beating hearts of their midfield.