The 23-year-old joined the Pilgrims earlier in the campaign on an emergency loan deal but had to return to Norwich after just seven games due to a leg injury.
Plymouth have brought the stopper back on another emergency deal but they are expected to extend his stay officially when the January transfer window opens.
Matthews is a product of Norwich’s academy, however, he is yet to make a league appearance for the Championship outfit and is not expected to force his way into contention this season.
The stopper, who has previously spent time on loan with Burton Albion, Doncaster and Hamilton Academical, is likely to start for Plymouth when they travel to Blackpool on Saturday.
Watford boss Javi Gracia does not think Abdoulaye Doucoure's form will be affected by his omission from France's latest squad.