I don't actually think his passing is what made us look better. I think its a combination of two things.
a. In possession - his willingness to provide an option for our defenders to pass to allowed us to get the ball up the pitch much easier. He consistently moved into the right spaces to receive the ball, which meant that we didn't need hollywood balls or touches to get the ball up the pitch.
b. Out of possession - His positioning in the opposition half meant that we couldn't be easily sliced open in the middle. This also made it difficult for Watford to progress the ball when we lost possession, as they would need to hit the ball at a longer distance to their forwards. This is something all the top teams have, that we don't have. This is what Hassenhuttl was talking about in his post match press conference. Fred and Mctominay do a lot of running, but their positioning and anticipation of both in the attacking half and defensive half are terrible. This usually makes it difficult for us to sustain good periods of play and makes it easy to counter us when we do lose the ball.
Matic doesn't have the legs to play well week in week out, but his performance showed the basics of what we need in our midfielders. Players who can recieve the ball, are not terrible at progressing it and can anticipate and place themselves in positions that make it difficult for the opposition to play through the middle. It would shock United fans to know this, but if we simply added Henderson into our midfield, we would look a whole lot better every week. These are the basics anyone playing in central midfield in England should have, since games are still won on second ball which means anticipation and positioning are still the key to winning a midfield battle. None of Pogba, Mctominay, VDB, Herrera, Fred or Fellaini have had these abilities.