The 25 year-old home fighter was given a 10-8 round for a suspect knockdown and another for alleged holding by the challenger, both of which seemed harsh on veteran Witter who seemingly had done enough to take the belt back to England.
Witter, 41, drops an eighth loss in 53 bouts with the head-scratching defeat and remains without a victory over an opponent with a winning record for almost three years. In contrast, champion El Mousaoui is now 22-1-1 and undefeated since 2011.
Elsewhere in Minnesota, Ismael Barroso took the frustration of not being able to face Derry Mathews for the interim WBA lightweight title on Saturday out on the hapless Ira Terry at the Grand Casino in Hinckley.
The big-punching 32 year-old was pitched in as a late replacement for Richar Abril to face Mathews after the Cuban fell ill, although Visa issues meant stablemate Tony Luis replaced Barros at the very last minute.
A massive left hand from the southpaw with just 20 seconds left of the first round ended any hopes Terry held of an upset as the hardened Venezuelan made it 17 stoppages from 18 wins through his 20-fight undefeated record.
Also on the card, there were impressive performances from Dennis Hogan and Jarrell ‘Big Baby’ Miller, whilst elsewhere in Connecticut, Dusty Hernandez-Harrison, Murat Gassiev and Ray Robinson enjoyed successful outings.