Midfielder Paul Hartley believes the new system, which has produced two victories, will stand the team in good stead for Champions League trips.

"There have been some good performances over the last few weeks," Hartley told celticfc.net.

"I felt we were really good against Southampton, we got a great win against Porto and the Feyenoord game was excellent.

"The different formation worked well against Feyenoord. We've used it in a few games now and the boys looked comfortable with the system.

"Myself and Massimo Donati sat deeper in midfield, and that allowed Scott Brown to get forward with Aiden McGeady and Georgios Samaras to support Jan Vennegoor of Hesselink.

"I'm sure it's given the manager something to think about for the season ahead.

"The shape was good and against certain opposition, especially in Europe, it could be really useful."