Former champ Johnson to miss British Open due to coronavirus
The 45-year-old American and De Jager are the latest stars to pull out of the British Open, which starts on Thursday at Royal St George's in Kent after being cancelled last year due to the pandemic.
Ryan Moore also withdrew from the event on Monday due to a back injury, taking the total number of players to opt out so far to 16.
Reigning Masters champion Hideki Matsuyama and US star Bubba Watson withdrew on Sunday due to Covid-19.
Adam Long, Sam Horsfield and Dylan Frittelli are now in the field, with Wade Ormsby, Matthieu Pavon and Ross Fisher next on the reserve list.