Mbabu made his senior debut for Newcastle against Chelsea, with the team short of options at left-back, and he shone throughout the match.
The youngster, who traditionally plays on the right hand side of the defence, was full of energy and desire, managing to keep the likes of Eden Hazard and Pedro quiet throughout the fixture.
His performance helped Newcastle pick up a 2-2 draw against the current Premier League champions, and Simpson has challenged the 20-year-old to maintain those high standards.
“I thought he was fantastic," he said to the Chronicle. "He looked totally unfazed by the game, totally unfazed by the opposition and the incredible noise our supporters made.
“Now the big test is ‘Can he do it again?’ Can he perform like he did and do the work he did?
“With high intensity and sprints, he outperformed everybody in that game. “Now is the real challenge – how does he deal with it? He’s got the preparation leading into the next game – if he’s selected, can he do the same thing?”
If Mbabu is given the nod again this weekend, his task is not likely to get any easier with a game against Manchester City next up for Newcastle.
City have a wealth of attacking options, and stopping David Silva, Raheem Sterling and Kevin De Bruyne has proven a tough test for numerous defenders this season.
It makes Newcastle's task of getting a result at the Etihad Stadium look a tough one, but Steve McClaren's men will realise that they cannot wait much longer to get their first win of the season, otherwise they could be in danger of being cut adrift in the relegation zone.