Scotland name U's midfielder in squad.
"Craig is on the other side of things, making sure we recruit well - he a young player that we saw massive potential in and one we wanted here at the club."
"Louis has come all the way through the ranks, studying and training at Thurstable before joining full time in the youth team, and has already made appearances in the first team.
"Craig and Louis are great examples of what we are doing here at Colchester United and it's great that the work we have done has also been recognised.
"It's the biggest accolade to play for your country and I'm pleased for the boys that they are been recognised by Scotland and Ireland," he told the website.
Whilst McGreal will be without two of his squad, he was pleased with the call-ups.
Youngster Louis Dunne could also be away from north Essex in the coming weeks, having been named on the standby list for the Republic of Ireland's U18s squad.
John McGreal will be without the twenty-two year old for the games at Crawley in the Checkatrade Trophy and the home league game against Exeter.
Slater will be looking to add to the five caps he already has for the Scottish U21s, but he will miss two U's fixtures while he is away.
The midfielder is in the squad for the U21s Euro Qualifying games against Macedonia and Ukraine in September.
Craig Slater has been called up by Scotland U21s.