Boss happy for Slater's Scotland opportunity.
"It's great for Craig - he's come down from Scotland to give English football a go, but he's still getting the accolade of playing for his country, so it's a real shot in the arm for him.
John said: "I think it's terrific for him. We got a call from Ricky Sbragia last week and we were more than happy for Craig to go and represent his country.
Craig is due to join up with his national teammates next week and John is pleased for his summer signing to get the nod from his country.
Colchester United Head Coach John McGreal has expressed his delight at Craig Slater's call-up for the Scotland U21s.