Six players have made the move south from the Scottish Premier League.
Stephen McManus: “It is very rare, I suppose, to move to a new club and find so many of your old team-mates here but straight-away everyone has settled in and you need that at a club.
“For me, personally, this is the first time I have moved away to a new club so it has been a massive help to have guys I played with at Celtic already here.
“It is important you do not have any cliques in the changing room, however and it is not going to be a case of all the Celtic lads sticking together.
“It is not just about the new lads who have come in but the ones already here. There are some top, top players here and they are young as well so hopefully, if we can pass on our experience we can make the team better. We are all pulling in the same direction.
"We are all in this together. It is not a case of a Celtic old boys but a new-look Middlesbrough.
“It was a good opportunity for me for a wee change and it was a chance to work with Gordon Strachan again."