As much as I agree that by the eyeball test, Robben, Pires and Giggs are players a class above Diaz, when it comes to the hard data on league goals scored, Diaz actually compares well to the lot except Pires. By the eyeball test, I would put Diaz around Duff's level.
Lucho arrived in the January transfer window and scored 4 goals in 13 league games. Then another 4 in 17 league games in his second season, a season where he lost 4 months to a knee injury. Post-injury, he scored 8 in 37, then 13 in 36 league games the last 2 seasons.
Giggs averaged around 6 to 8 league goals in his peak seasons, with highs of 11 and 13, and lows of 1 and 3.
Robben? 7, 6, and 2 goals in 3 seasons, 2 of which were injury-blighted.
Duff ... averaged 6 to 7 goals at Blackburn, with a couple of 1-goal seasons. Then 5, 6 and 3 at Chelsea.
Pires was a different class now. 14 goals a season consistently for 3 seasons in a row, bookended by seasons with 9 and 7.
But again, I agree that by the eyeball, I'd pick Giggs, Robben and Pires over Diaz. Then I look at the data, and I think, maybe Diaz wasn't that far off from them. We might have negative biases when we watch our own guys because we see and feel the flaws often and closer, whereas with the others, we catch highlights, or some of their full games, and we think they poop gold.