Chelsea are set to sign Senegal defender Kalidou Koulibaly from Napoli on a four-year deal.
The 31-year-old is set to travel to join the squad in Los Angeles on Wednesday before having a medical.
Chelsea are also close to bringing in Manchester City centre-back Nathan Ake.
The pair would bridge the gap left in central defence after Antonio Rudiger and Andreas Christensen left the Blues this summer for Real Madrid and Barcelona respectively.
Earlier on Wednesday, Chelsea confirmed the signing of England forward Raheem Sterling from Manchester City in a £50m deal.
Koulibaly, who has attracted interest from several Premier League clubs, joined Napoli from Genk in 2014 and has made 317 appearances for the Italian top-flight side.
Chelsea are training at the University of California, Los Angeles (UCLA) and Sterling could be available to play in their opening tour match in Las Vegas on Saturday against Club America.
Tuchel's men then face Charlotte FC in Charlotte on Wednesday, 20 July before taking on Arsenal in Orlando on 23 July.
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