He was a United youth product that was starting to make some inroads into the first team, I believe he had a CL squad number and might have played a few mintues with the senior setup. He ran down his contract so was free to leave.
Spurs wanted to sign him, but as part of the English rules, if an English youth product is signed who is under 23 then the academy that developed him is due a fee (normally reasonably small too). So Spurs had him sign for 6 months with Standard Liege, then bought him from there.
It was a glorious dodging of the rules from Levy, and more worrying for us (especially with Ed's ability to sell people) it opened a loophole that is supposed to protect English academies (yes yes, Rossi/Pogba/etc etc) and as the largest producer of English players playing in the premier league, that's a big deal to United.