Yamal, 18, starred for the La Liga champions last season, scoring nine goals and assisting 13 in a breakout campaign.
He had started the new season on fine form, too, registering four goal contributions in his first three league outings, with three assists in two appearances for Spain to boot.
But the young Spaniard's momentum was checked over the international break as a pre-existing groin issue resurfaced.
This latest recurrence is expected to leave him sidelined through the end of the month, with reports in Mundo Deportivo saying he may not be fit in time for the Blaugrana's trip to face Real Oviedo on September 25.
It is a major boost for Eddie Howe's Newcastle, who will be hoping for improved fortunes in the Champions League this season after crashing out in the group stage in 2023-24.
Yamal is yet to train with Barcelona since returning from international duty and was not named in the matchday squad for Sunday's 6-0 win over Valencia.
He missed training while with the national side and received painkillers so as to appear in World Cup qualifiers against Bulgaria and Turkey, which yielded 3-0 and 6-0 wins respectively.
Speaking ahead of the Valencia match, head coach Hansi Flick said he was 'very sad' not to have Yamal available.
"He won't be available," said Flick last week.
"He went off to play for Spain with some pain and did not train. They gave him painkillers so he could play. They had at least a three-goal lead in both matches, and he still played 73 and 79 minutes," Flick said last week.
"Between the games, he wasn't able to train. That's not looking after the player. I'm very sad about this."
The player is reportedly suffering from pubalgia, a chronic groin injury which can recur if not properly treated. Barcelona are expected to take a conservative approach to his treatment and will not risk rushing his return.
Barcelona are also set to be without Alejandro Balde, Gavi, and Marc-Andre ter Stegen. Frenkie de Jong is a doubt after leaving the Netherlands camp early with an injury, but has returned to training ahead of the match.