Louis van Gaal’s side suffered a nightmare start to the game, going behind thanks to a Michael Carrick own goal after just eight minutes at Old Trafford.
However, Memphis levelled just five minutes later after beating a couple of defenders before firing past goalkeeper Sebastien Bruzzese then adding a superb second from range.
The 21-year-old had further opportunities, in particular firing over the bar after a great team move involving Luke Shaw and Juan Mata and he rues his misses.
“I am happy of course that I scored and we won the game. We created a lot of chances,” he told BT Sport.
“It really important to score the last [third] goal of course but I’m a little disappointed that I didn’t finish the last two chances. It’s a good result and we have to travel to Bruges and finish the job.
“At the end of last season I finished with a lot of goals and to come to a big club you have to score and do the job. It’s a nice feeling to score at Old Trafford
"I am little disappointed because I will think about that last chance."