The Formula Renault 3.5 driver is taking on the reserve role at this weekend's Chinese Grand Prix.
Magnussen is the son of former McLaren driver Jan and part of the outfit's young driver programme.
Paffett has not travelled to Shanghai as he has had DTM testing commitments at Hockenheim.
Turvey is racing at Silverstone in the European Le Mans Series.
McLaren has confirmed that Magnussen, Paffett and Turvey will share the third driver role for the remainder of the year.