Update Aug. 18, 4:15 p.m. ET: Zack Wheeler underwent a successful thrombolysis procedure to remove a blood clot on Aug. 18.

The Philadelphia Phillies’ World Series chances just took a big hit after the recent injury to Zack Wheeler, who’s on the 15-day injured list with a right upper extremity blood clot with a return-to-play date “unclear,” as ESPN’s Jeff Passan put it.

Once again, Wheeler was in the midst of another Cy Young-caliber year, with a 10-5 record and 2.71 ERA in 24 starts to go with an MLB-best 195 strikeouts. However, over his last six starts, something had looked off with the 35-year-old ace.

Since a one-hit complete game against the Cincinnati Reds on July 6, Wheeler has allowed two or more runs in each of the following starts thanks to worse command and a downtick in velocity. Now we know why.

Zack Wheeler’s symptoms & the different types of blood clots

While the exact type of blood clot that Wheeler is dealing with hasn’t been confirmed yet, the two most common blood clots in the shoulder region are deep vein thrombosis (DVT) and a type of thoracic outlet syndrome (TOS) called venous thoracic outlet syndrome (VTOS).

DVT can be attributed to some form of strenuous activity or events such as rowing or pitching. Symptoms typically include hand weakness, swelling of the hand or arm, pain that travels to the hand or forearm, or shoulder pain.

Meanwhile people can develop TOS from certain repetitive actions that irritate or injure the structures in your thoracic outlet, which is located in the upper chest right above your first rib and behind the collarbone. Repetitive motions like throwing a baseball is one of the most common causes of VTOS and symptoms include heaviness, aching or swelling in one arm or hand, mainly the dominant one.

After his outing against the Washington Nationals on Friday night, Wheeler felt “a little heaviness” on his right shoulder after the game, according to Phillies head athletic trainer Paul Buchheit, per Todd Zolecki of MLB.com.

“Zack had been feeling better after his right shoulder soreness,” Buchheit said, per Zolecki. “But yesterday, some symptoms had changed. Doctors were great in helping to diagnose and expedite that diagnosis this morning.”

Treatment options & return-to-play possibilities

Despite the difference between DVT blood clots and TOS ones, the treatment plans are fairly similar and include both nonsurgical and surgical options.

If the clot isn’t too severe, then rest may break up the blood clot. But typically, doctors will prescribe blood thinners in attempts to break down the blood clot. However, the patient is expected to stay on these blood thinners for a minimum of three months, thus ending Wheeler’s season.

Then, depending on how the body responds, a type of surgery called first rib resection may be in play. This surgery is a common treatment for thoracic outlet syndrome and is done by removing the first rib and any other surrounding structure or scar tissue to help alleviate the symptoms of TOS.

If Wheeler does undergo this operation, the standard return to play timeline is roughly 10 months after the date of surgery, which would bring us to approximately the middle of June 2026.

Other MLB pitchers who have dealt with TOS

Wheeler isn’t the first pitcher to deal with this type of injury and surely won’t be the last. Just as recently as 2020, former Arizona Diamondbacks starting pitcher Merrill Kelly underwent surgery to combat his vascular TOS in September 2020. He returned to the mound during spring training of 2021.

More famously, the tail end of former Washington Nationals starter Stephen Strasburg’s career can be defined by a long battle with TOS. However, Strasburg had neurogenic TOS, which was much more extreme compared to Wheeler’s case.

Other examples include Chris Carpenter in 2012, Jaime Garcia in 2014, Chris Young in 2014, Matt Harvey in 2016, and Chris Archer in 2020, along with many others.

All of these pitchers made it back to the mound the following year, and the data shows that there doesn’t seem to be any performance drop-off, according to the Orthopedic Journal of Sports Medicine. In a time when there’s so much uncertainty around the health of the Phillies’ ace, that’s all you can ask for if you’re the Phillies or Wheeler.