The pair previously played together at Leeds United.
The 19-year-old midfielder more than held his own against a star-studded Red Devils side yesterday, even though the result was not fantastic.
It was Cook's first competitive outing for Bournemouth since leaving Leeds in a potential £10 million deal (according to The Guardian) and his performance was certainly the overriding positive on an otherwise disappointing day for the Cherries.
Cooper spent two years with Cook at Elland Road, and the centre-back is "buzzing" to see his friend dazzle on the big stage.
Well done my man! Buzzing for you.. https://t.co/BugWtJGgQb— Liam Cooper (@LiamCooper__) August 14, 2016
It is very early days but, given the Jose Mourinho and Zlatan Ibrahimovic factor, the world will have seen how promising a talent Cook - last season's Young Player of the Year in the Championship - is and it will be a huge confidence boost for the teenager.
Things haven't been going well back at Leeds, who lost their first two Championship games to put a real downer on their start to the season after a very optimistic summer.
Manager Garry Monk brought in eight new players but unless their form picks up rapidly then the 38-year-old may not be around to help them develop.