In his second season back from Tommy John surgery, Harvey has struggled overall, leading the National League with 10 losses and posting a 4.86 ERA. But after his ERA ballooned as high as 6.08 on May 24, Harvey posted a 2.27 mark in six subsequent starts prior to Tuesday's no-decision.
Aside from a period of general arm fatigue early last season, Harvey has had no documented medical issues since returning from surgery in April 2015. He made 29 starts last season and another four in the postseason, throwing a combined 216 innings -- a career high, eclipsing his previous mark by 37 2/3.
The trip to St. Louis is unorthodox for a Mets team that sends its pitchers to its own team orthopedist, Dr. David Altchek, for routine examinations. Thompson is not an orthopedist, but a specialist in "all forms of thoracic outlet syndrome and upper extremity neurovascular compression disorders," according to his website. In 2012, former Mets pitcher Dillon Gee notably visited Thompson, who diagnosed him with a damaged artery and performed season-ending surgery on him. Another former Met, Chris Young, visited Thompson in 2013 for surgery to correct thoracic outlet syndrome, a nerve condition.
A Mets spokesman said the team does not plan on releasing any more information until after Thompson issues a diagnosis.
But it is nonetheless worrisome for a club with thinning pitching depth. Starters Noah Syndergaard and Steven Matz are both currently pitching through bone spurs in their elbows, though Matz's is much more significant. Zack Wheeler has experienced multiple setbacks in his rehabilitation from Tommy John surgery, and may not make it back to the big leagues until August. If Harvey misses significant time, the Mets will need to turn either to Logan Verrett or Sean Gilmartin, both of whom have made spot starts this season, or dip into the Minors for another solution.
No. 13 prospect Gabriel Ynoa has been the Mets' best starting pitcher at Triple-A Las Vegas, going 9-3 with a 3.92 ERA over his first 17 starts.
To replace Harvey on the active roster, the Mets recalled No. 22 prospect Seth Lugo, who threw two innings for the Mets in his big league debut last week. In 13 starts and six relief appearances at Vegas, Lugo was 3-4 with a 6.55 ERA.
Anthony DiComo has covered the Mets for MLB.com since 2008. Follow him on Twitter @AnthonyDiComo and Facebook, and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.