Barcelona’s players were put through their paces at England’s St George’s Park training base on Monday as they began preparations for their opening pre-season friendly against Celtic on Saturday.The La Liga champions, visiting England’s training headquarters for the second time, will spend six days at St George’s Park before flying to Dublin to take on Brendan Rodgers` side at the Aviva Stadium in the International Champions Cup, live on Sky Sports 1 HD. Lionel Messi and Luis Suarez were among the headline names in Luis Enrique’s 25-man squad, which included 12 players from Barcelona B and their U19 team. Messi had not been expected to make the trip to England after being afforded an extended period of rest by Barcelona following his exertions at this summer’s Copa America, where he helped Argentina reach the final.However the 29-year-old, who has since retired from international football following the final defeat to Chile, cut short his break and was seen getting straight back into the thick of things on the training ground. Cristian Tello, Martin Montoya, Jeremy Mathieu and Arda Turan were also back in training for Barcelona, but Gerard Pique, Andres Iniesta and Ivan Rakitic were absent after being given additional time off following Euro 2016.