Celtic played a back four with Tierney. Tierney stepped forward, Lustig tucked in with Brown now playing in front of a back 3. The left winger moved in field to play off the striker as Tierney now occupied his role. If you were to class this as a formation then it would have been something daft like 3-1-2-2-2. When Celtic would be defending everyone would then return to their starting positions in a 4-2-3-1.
Celtic did ok.