It is understood the two midfielders are on Emery's radar as he accepts the Welsh star is set to leave the club.
Herrera, 28, is the Porto captain and, like Ramsey, his contract at his current side runs out next summer.
It means he would be able to move for a cut-priced fee in January or can negotiate a pre-contract Bosman agreement to move at the end of the season.
Arsenal sent a scout to watch him in action for his club in the Champions League last week.
The Gunners are also set to sign Atlanta United star Almiron in January.