Barclay, who has 41 international caps to his name, had initially been called up for the quadrangular tournament in South Africa, which also features Samoa and Italy.
But he will now undergo an operation later this week. Despite this, Wilson believes the 26-year-old will be fit and raring to go by the time the new season gets underway.
Wilson told Wales Online: "He will be back for the start of the season - it's just a tidy up of a shoulder and is nothing serious.
"John is a good signing, what we needed.
"He's a leader, a seven who has played at the top end of the game.
"We need and want to sign a No. 8, but, at the same time, it's got to be the right person to do a job," added Wilson.
"What I have noticed about the Scarlets since I joined them a year ago is they are very thorough in the process they go through before making a signing.
"You can never be 100 per cent certain of success with signings, but Joe Snyman and George Earle have been good for us."
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