West Ham have announced the signing of Swiss midfielder Edimilson Fernandes from FC Sion.
Sources close to the club told ESPN FC on Wednesday that the teams had agreed a £5.5 million fee.
The 20-year-old was introduced on the pitch of the London Stadium before Thursday evening's Europa League qualifier against Romanians Astra Giurgiu.
Walter Mazzarri has spoken of the importance of his new signings but may not start them against Arsenal on Saturday.
Daryl Janmaat arrived on Wednesday from Newcastle for £7.5 million, shortly after last week's recruitment of Younes Kaboul and club-record signing Roberto Pereyra.
A weakened team lost 2-1 to Gillingham in the EFL Cup on Tuesday, but Mazzari was sufficiently impressed by his players' performance in Saturday's defeat by Chelsea via the same scoreline he is considering giving them...