Phillies ace Zack Wheeler underwent a thrombolysis procedure Monday morning to remove a blood clot near his right shoulder.

The Phillies said Wheeler had the procedure, performed by Dr. Paul DiMuzio, at Thomas Jefferson University Hospital. The club said Wheeler's timeline to return and further treatment options are to be determined.

"The clot was cleared out," Phillies manager Rob Thomson said. "We don't know until we get further information. A lot of people ask me about the pitching staff and the team, and right now, my thoughts are just about him.

"I said the other day, this isn't like a hamstring or a calf, this is real. This is life," Thomson added. "So, my thoughts are constantly on him and his family, and hopefully everything works out. So far, so good."

Wheeler, 35, was placed on the 15-day injured list Saturday with a "right upper extremity blood clot." He started in Washington Friday night, allowing two runs in five innings. He had been dealing with right shoulder soreness for over a month.

After the game Friday, Wheeler reported feeling a "little heaviness" in his shoulder, head athletic trainer Paul Buchheit said Saturday.

Buchheit said Saturday that Wheeler's blood clot was independent of his previous stiffness.

What is thrombolysis?

Thrombolysis, the procedure the Phillies' ace underwent, removes a blood clot with a catheter. Patients who undergo this type of procedure are generally put on blood thinners and are advised to rest for three months.

Dr. Michael Lieb, a vascular surgeon with Virtua Health who's not involved in Wheeler's case, spoke with CBS News Philadelphia about the treatment.  

"This is usually a needle-based method," Lieb said. "Just like getting an IV or giving blood, the needle goes into the vein. There are different types of catheters to be used to pull the clot out.

"After a procedure like this, we still typically continue patients on blood thinners for several months afterward," Lieb added. "As far as his activity level, we do usually recommend that they rest and not do repetitive activities."

What's next for the Phillies?

The Phillies are now without Wheeler for the foreseeable future amid another Cy Young-caliber season. Wheeler has a 2.71 ERA and leads the National League in strikeouts with 195 across 24 starts.

"He's an ace. He's the heart of this team, the heart of this staff," Phillies reliever Matt Strahm said Monday. "You never want to see it. He's a dad. His family comes first. That's what we worry about mostly."

Wheeler anchored MLB's third-best rotation. Now, the club will move forward without its ace. Aaron Nola returned Sunday after missing months with a sprained ankle and fractured rib. It didn't go well, as Nola allowed six runs in 2 1/3 innings.

Thomson said Monday the Phillies will move forward with a five-man rotation, meaning, for the time being, top prospect Andrew Painter's debut will have to wait.

"We're running out the normal rotation right now," Thomson said.

Ranger Suárez, Cristopher Sánchez and Jesús Luzardo are the probable starters for the Phillies' three-game series against the Seattle Mariners beginning Monday. Taijuan Walker lines up for Friday and Nola again on Saturday against the Nationals.

"Zack's a great pitcher, and you can always rely on him every fifth day, but at the same time, I have confidence in our entire rotation," Thomson said. "Our bullpen is much better now, so as tough as it is to lose Zack, I feel really good about our ballclub. I really do."

"As I told everybody, just be yourselves. That's all we can do," Thomson added. "Don't try to be anybody else because they're good enough just the way they are."

More pitching roster moves on the way

Thomson said lefty reliever José Alvarado will be activated Tuesday, the first day he's eligible to return from his 80-game suspension for testing positive for a performance-enhancing substance.

The Phillies also recalled right-handed pitcher Nolan Hoffman from Triple-A Lehigh Valley and optioned Max Lazer to the IronPigs.