Of the three games you mentioned, Pereira didn't play against Everton or Watford, and Villa was one of those two games he had in central midfield when nobody else was fit and he was absolutely terrible. Obviously we should have a better starting #10, but equally obviously he's been our best this season. Not amazing, but he's been decent (which instantly puts him significantly ahead of Lingard and Mata).
You got me interested so I actually just looked up the stats and they make interesting reading:
Pereira has started 7 premier league matches this season at #10. We've won 5 of them. The two matches we didn't win included the 1-1 against Liverpool where we were the only team this season to take any points off them. So that means only 1 match out of 7 where we wrongly dropped points (the 1-0 loss against Bournemouth). 16 points out of a potential 21, or 2.28 ppg (points per game).
In the 5 premier league matches where he started on the right wing, we've had 2 wins, 2 draws and a loss. 8 points out of a potential 15, or 1.6 ppg.
In the other 10 matches (which include him playing at CM, him coming on as a late sub, or him not playing at all) we've had 2 wins, 4 draws, 4 losses. 10 points out of a potential 30, or 1 ppg.
As I said, it makes interesting reading. In comparison, Mata's numbers at #10 are 1 win, 2 draws and 2 losses (1 ppg), and Lingard's are 3 wins, 3 draws and 3 losses (1.33 ppg).