The 22-year old Kenyan midfielder has completed his £12.5million shift to St Mary's and was quick to thanks the Scottish giants who helped develop his game over a successful two-year stint.
"It has been hard. Celtic have been good to me and the fans have been great. To be honest it was hard leaving them, but I wish them all the best," he told Sky Sports News.
"I had a good time with them. I want to thank them for the time I was there and hopefully one day we will be back.
"I think Celtic have played a big part in my improvement and I want to thank the manager and the players. It was really good and I enjoyed it."
He added: "I'm excited. It's a new challenge and I'm looking forward to it.
"I need to go and work hard and try to get in the team. We will see what happens from there."