Celtic midfielder Scott Brown thinks Stuart Armstrong will be a big loss - but expects him to thrive with Southampton.
Armstrong has signed a four-year deal with Southampton - having helped Celtic to claim eight major trophies, including back-to-back domestic trebles.
And Brown told Sky Sports: "We wish Stuart all the best - he has been fantastic for this club when he has played.
"Last season it was hard for him, a few injuries, and coming in and out of the team.
"But Stuart is a fantastic player, he knows what I think of him personally, and he will be a big miss for this club.
"He will relish the chance to go down to England to do well and I am sure he will do."
Meanwhile, four members of the Celtic squad - Dedryck Boyata (Belgium), Mikael Lustig (Sweden), Cristian Gamboa (Costa Rica) and Tom Rogic (Australia) - have been on duty at the World Cup in Russia.
Boyata and Belgium take on in-form England striker Harry Kane on Thursday night.
Brown added: "It is fantastic for all the lads and they have all done really well, when I have watched them.
"It is great for Celtic as well.
"'Deddo' (Boyata) has played against top quality players like (Lionel) Messi and I am sure he won't have a problem with Harry Kane!"