Wissa joined Newcastle in a £55m move from Brentford on deadline day at the start of this month, but suffered a knee injury while playing for DR Congo last week.
Newcastle’s medical staff assessed the injury when the striker returned to Tyneside, but recommended a visit to a specialist to confirm the full extent of the problem.
Wissa met a specialist and had a series of scans, and has suffered damage that is expected to keep him on the sidelines for around four-to-five weeks.
The 29-year-old is not expected to be available until the end of the next international break, which runs from October 6-17.
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That means he is set to miss Newcastle’s next six matches – three league games against Bournemouth, Arsenal and Nottingham Forest, two Champions League matches against Barcelona and Union Saint-Gilloise and a Carabao Cup game against Bradford City.
The news is a major blow to Magpies boss Eddie Howe, although it is not as bad as some of the worst-case scenarios that might have unfolded after Wissa suffered knee damage.
Rumours were circulating today that the forward had suffered ACL damage, which would have ruled him out of the vast majority of the season, but that is not the case.
Wissa has not suffered major ligament damage, but Newcastle’s medical staff will be understandably cautious as they over see his recovery.
With Wissa sidelined, Howe will have to rely on Nick Woltemade, who scored on his Magpies debut against Wolves, and Will Osula as his main two centre-forward options.
Anthony Gordon could also play as a central striker once he returns from his domestic suspension. Gordon is available for Thursday’s opening Champions League game against Barcelona, but will complete his Premier League suspension when he sits out next weekend’s tip to Bournemouth.