It follows their opening day defeat to Hearts in the Scottish Premiership.
Republic of Ireland midfielder James McCarthy has signed a four-year contract at Celtic.
The 30-year old was a free agent after his Crystal Palace contract expired.
Also joining Celtic today is former England and Manchester City goalkeeper Joe Hart.
π ππ’π¨πππ ππππ! ππ
— Celtic Football Club (@CelticFC) August 3, 2021
Welcome to #CelticFC, Joe Hart and James McCarthy! π#WelcomeHart π€ #WelcomeMcCarthy