Elanga has been omitted from Graham Potter's Sweden squad after suffering the problem over the past week, with the 24-year-old now set to miss matches against Romania, Poland and Bosnia and Herzegovina.
The winger's absence is another setback for Newcastle after he made an impressive start to the season under head coach Matthias Jaissle. Elanga scored twice in his first three Premier League appearances before missing Sunday's trip to Leeds, where Newcastle suffered a 4-1 defeat.
Jaissle revealed before the match that the injury could potentially keep Elanga out for an extended period, although further tests are required to establish the full extent of the problem.
"Unfortunately he is injured," Jaissle told Sky Sports. "We need to investigate further and in more detail to find out how long he will be out. We will miss him and hope he is back to play quickly.
"It could be (long-term). We hope not. He will get another MRI next week and we will see."
The Swedish FA subsequently confirmed that Elanga would miss the international break, adding to the uncertainty surrounding his return date.
Sweden are preparing for Nations League matches against Romania in Stockholm on 25 September before hosting Poland three days later. Bosnia and Herzegovina are also in their Division B group.
Potter has selected an enlarged squad as he manages several injury absences, with Newcastle defender Isak Hien and goalkeeper Jacob Widell Zetterstrom among those unavailable.
The Sweden manager has also opted to look towards the future in goal, leaving veteran Kristoffer Nordfeldt out and selecting younger keepers Melker Ellborg and Noel Tornqvist.
"I think it's a good time to look a little forward, bring in more younger goalkeepers who we need to get to know and gain a greater understanding of, while giving them the chance to gain experience at this level which is important for their development," Potter said.
Despite the absences, Sweden can still call on Alexander Isak and Viktor Gyokeres as they prepare for the September fixtures.
For Newcastle, however, the immediate concern is Elanga's fitness and whether the winger's latest injury will keep him out beyond the international break.
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