World number one Johnson, who walked off the first tee and withdrew from the Masters a day after slipping down a staircase and injuring his lower back, had been favourite to win at Augusta National.
The 32-year-old American, who won last year's U.S. Open and took over as the world number one in February, had arrived at the Masters having won each of his previous three events.
Johnson's best finish in the Wells Fargo Championship was a tie for 29th in 2010. In his two other starts, he missed the cut.
There are currently six players among the top-20 in the world now in the Wells Fargo field as Johnson joins Australian Adam Scott, Swede Alex Noren, Spaniard Jon Rahm and the US pair of Patrick Reed and Phil Mickelson.
Players have until April 28 to commit to the field.
(Reporting by Frank Pingue in Toronto; Editing by Ian Chadband)
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