Mourinho has now failed to win at St James' Park in all seven of his Premier League visits, while the result surely ends any residual hope of a late title push.
Mourinho's side remain second, 16 points adrift of Manchester City, but the Portuguese claimed to draw consolation from his old mentor, the former Newcastle manager Sir Bobby Robson.
After a bright start from Newcastle, United began to get more of a foothold in the game, with Sanchez in particular looking capable of producing something, but his early link-up play with Martial and Lukaku didn't quite work out.
The Magpies gave a very good account of themselves in the first half, in which neither side were able to score, in part due to a confident debut from Newcastle's new Slovakian goalkeeper Martin Dubravka, a recent arrival from Sparta Prague.
"Newcastle's players gave the sacrifices all managers like to see".
Ritchie's victor came from a clever flick by Dwight Gayle after Jonjo Shelvey's lofted free-kick had been headed down in the area. The visitors haven't lost away to Newcastle since 2012. Alexis had an open goal and then the countless opportunities of the last 20 to 25 minutes.
Huddersfield's first league win in nine matches was marred by an injury to Australia midfielder Mooy, who was stretchered off with a knee problem 15 minutes from time. From the set piece, the ball was nodded down to Dwight Gayle, whose flick was met with a first-time finish by Ritchie. A central midfielder should never be in areas wider than the penalty box, unless they are out there to confront danger defensively. After Chris Smalling conceded a free-kick for diving near the halfway line, Shelvey sent a simple long ball into the box, Gayle cushioned a loose ball into Ritchie path and he swept home completely unmarked from 12 yards out.
The England worldwide has been at least a substitute in every match in all competitions under Jose Mourinho in 2017-18, although he did not get off the bench in their last away game, when they lost 2-0 to Tottenham.
David Wagner's team took the lead in the seventh-minute through Alex Pritchard, only for Junior Stanislas to level for Bournemouth with a cool finish from Ryan Fraser's cross.