I have been flamed elsewhere in this thread for his ridiculous appearance against PSG.
But ok, let's turn it around, let's forget he has no spatial awareness and passes and runs into the wrong places. What does this guy have in your opinion? Is he strong? Is he fast? Can he dribble past an opponent? Does he have a dangerous shot? Does he have these smart assists? What does he excel in? Why is he playing for United?
If you watched Chong play in either United's U18s, U19s or U23 teams, he has been one of the top two or three in the respective squads each time (Edit Thurday. He just won the Denzil Haroun U23 player of the season. Last season he won the Jimmy Murphy award for best U18 player). That might make him the best out of a group of young players who won't become PL standard players, or he might be over-taken by a lesser talent that develops more over the next few years. Right now, Greenwood, Chong, Gomes and Garner are the young players that the club feels have the best chance. Kieran McKenna who was the U18 coach when Chong played at that level, is now one of the first team coaches, so I assume he will have had a major say in the decision to promote Chong.
United have a 25 player first team squad. None of the youngsters are currently in the 25. They typically only appear in the match day squad when there are several injuries. None of the four have started a game so far.
Chong has 76 mins in total, during which time he has run about a lot, given the ball away carelessly rather too often, had a few promising moments but generally looked very raw. Personally, I'd rather wait until he has 1000+ mins, perhaps 2000 mins, in the first team, ideally mostly Europa League and domestic Cup competitions, rather than PL, before making an assessment. Even then, I'd be cautious about how he might develop in his mid 20s. He could improve a lot yet still not be good enough for United.
United have to take risks. There is risk involved in signing a top player like Sanchez or Mkhitaryan, then hoping they can bring their elite performances with them from their old club, from a set-up that likely suited their strengths and weaknesses. It could be the risk of buying Memphis Depay or Lindelof, players that impressed in a weaker League. Or it could be the risk involved in promoting a few raw kids and hoping they can develop sufficiently to become first team squad players within a reasonable time-frame.
During my time watching United, Macheda made his debut in a crucial PL game late on in the season and went on to score a spectacular late goal that won the match, helping United towards the title. He left United after several underwhelming loan spells, played in the Championship for a few years, then Serie B, currently he's playing in Greece. The much hyped Ravel Morrison, the outstanding youth player since the Class of 92, is another for whom the early predictions turned out to be wildly inaccurate. No-one on this forum, indeed barely anyone at United apart from Mourinho, thought McTominay had even the slightest chance of becoming a PL standard player, yet less than 2 years after his debut, he was United's best performer in a game against Barcelona, having performed well in the win at PSG in the previous round. Jesse Lingard in the England team that reached the World Cup semi-finals, is a bet that would have made you a lot of money, had you invested when he was in his early 20s. I had Januzaj down as a future elite talent after he had less than 10 starts for the first team; I still find it difficult to understand how he has barely improved since then.
You asked what does Chong have ? He's fairly tall for a wide player. He's quite fast. He seems brave. He takes risks. He's left-footed. He has scored some goals at U23, U19 and U18 level. He had a few promising moments during the first team's pre-season tour. I have no idea whether 24 year old Tahith Chong will be playing CL football, Keuken Kampioen Divisie, or some level in between.
I didn't have the slightest problem with United giving McTominay starts in the domestic Cup competitions and against weaker PL sides at home, to see if he would develop into a decent player. Similarly, I don't have an issue with United investing some time in Chong and seeing how things go over the next three or four years.