Celtic’s pre-season will ramp up in intensity and difficulty over the next three weeks after a pretty low-key start in Scotland. There’s no training camp in Europe this summer with the Bhoys deciding to stay back in Scotland before embarking on a three-game stint in the USA. The full squad has yet to even return to Lennoxtown after international matches and holidays. Two friendlies have come and gone against lower-league Scottish opposition. The biggest date on the pre-season calendar is a champions vs champions clash against Manchester City at the 50,000-capacity Chapel Hill stadium on July 2…