After spending three months recuperating from a knee problem sustained in pre-season, the Japan international finally made his first appearance of the season in Arsenal's 3-1 win over Southampton two weekends ago.
Tomiyasu replaced Thomas Partey for the final six minutes of that come-from-behind triumph, although he was not called up to the Japan squad for this month's World Cup 2026 qualifiers against Australia and Saudi Arabia.
There was no mention of a fresh issue for Tomiyasu immediately after the Southampton clash, and the 25-year-old was expected to be in contention for a start in this Saturday's clash with Bournemouth.
However, TEAMTalk claims that the injury-riddled Tomiyasu has potentially suffered a recurrence of his previous knee issue, and he could face another month on the treatment table.
Arsenal 'seriously concerned' about new Tomiyasu blow
Gunners staff are supposedly 'seriously concerned' about the former Bologna man's condition, and he is believed to have sustained the complication during his six-minute run-out against Southampton.
Tomiyasu's latest blow comes at an inconvenient time for Mikel Arteta, who is already stretched thin in the full-back areas thanks to Oleksandr Zinchenko, Ben White, Kieran Tierney and Jurrien Timber's injuries.
Arsenal have also been dealt a number of fresh blows over the international break, as Bukayo Saka hurt his hamstring while playing for England, and Gabriel Martinelli is confirmed to be nursing a calf issue.
In addition, Germany's Kai Havertz and Ghana's Thomas Partey both had to withdraw from their national teams, but Arsenal are apparently confident of having them back in time for Saturday's trip to Bournemouth.
Saka is also said to be optimistic about his chances of being fit for the clash with the Cherries, but Martinelli is more of a doubt, and captain Martin Odegaard is still missing following reports that he has also suffered a fresh setback.
What games will Tomiyasu miss? Should Arsenal consider selling him?
Should Tomiyasu not be available again until after the next international break, the Japan international stands to miss at least seven games for Arsenal, starting with Saturday's showdown against Bournemouth and the Champions League clash with Shakhtar Donetsk next week.
Arsenal will also have to cope without the versatile defender for the home tie against Liverpool on October 27, in addition to their EFL Cup fourth-round tie with Preston North End on October 30.
A critical 10-day period follows for the Gunners after that, as they meet Newcastle United (November 2), Inter Milan (November 6) and Chelsea (November 10) before the final international break of the year, and Tomiyasu should not feature in any of those games.
The 25-year-old is a terrific option to have available when fit - hence Arsenal rewarding him with a new deal earlier this year - but his injury record is now too difficult to ignore, and the Gunners must surely consider offloading him in 2025.
Arsenal are understood to be open to January or summer bids for Tomiyasu, who is out of contract in 2026 and is thought to be on the radar of Serie A giants Juventus and Inter Milan.