The 19-year-old midfielder was forced off during the second half of Spurs' 2-0 Champions League win on Tuesday night after a heavy tackle. Bergvall tried to continue but was soon replaced by 17-year-old debutant Jun'ai Byfield, with concerns clear immediately.
Speaking after the match, Frank confirmed the nature of the setback but stopped short of offering an instant timeline.
"It was a contact injury and they are assessing him now, so we will know more on Thursday," Frank told TNT Sports.
Subsequent assessments, backed by reports from Sweden and England, revealed Bergvall had suffered a high ankle sprain. The expected recovery time is between two and three months, a significant blow for a player who has struggled for continuity this season.
The injury is likely to sideline Bergvall for as many as 15 matches, depending on Spurs' progress in Europe. Tottenham are currently on course to qualify automatically for the Champions League last-16 in March, a stage Bergvall is now set to miss.
The setback also affects the international calendar. Bergvall is expected to miss both legs of Sweden's World Cup play-off semi-final against Ukraine at the end of March, and any potential final against Poland or Albania.
Bergvall's absence adds to Tottenham's growing midfield concerns. James Maddison (ACL injury (long-term), Mohammed Kudus (Thigh/quad tendon injury), Dejan Kulusevski (Complicated knee injury) and Rodrigo Bentancur (Hamstring injury) are already sidelined, while Joao Palhinha's (Knock injury (short-term)) fitness remains uncertain after missing the last two matches with a training-ground knock.
Elsewhere, Ben Davies (Ankle fracture/injury) is expected to miss most of the season following ankle surgery, and Richarlison is facing around six weeks out with a hamstring injury.
For Bergvall, it is another frustrating chapter in an injury-hit campaign, and for Frank, another test of Spurs' depth at a crucial stage of the season.
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