Neil Lennon's main concern will have been that they return injury free. But he knows what a boost a club can get if a player has done well with his country.
And, with Emilio Izaguirre and Fraser Forster already looking towards next summer's World Cup finals, and the likes of Georgios Samaras, Mikael Lustig and Efe Ambrose in line to play in next month's play-offs, there will be a good feeling about the place.
However, I know that, after spending time with your country where the training and the targets are different, the difficult part is how to adapt back to playing for your club.
When Scotland beat Czechoslovakia to qualify for the 1974 World Cup, I remember vividly how tough I found that, and I was not alone, as Celtic's defeat to St Johnstone three days later confirmed.
Big Jock was not a happy man and hammered home to us how important it was to get our mind back on the here and now and not be distracted by what we had done or what lay ahead in Germany the following summer.
The current Celtic squad has so many internationals from so many different countries that it is vital Neil gets them all back together, lets them talk about what has happened to them, then gets their focus on what they must now do as a Celtic team.
He will demand they pick up where they left off beating Motherwell in the game before the break when they play Hibs today because he will want to confirm all is well with the team before facing Ajax on Tuesday.