The MRI took place in between his first and second start in San Francisco, and it has prevented Wheeler from using his slider, the source told Puma.
According to Puma, Mets assistant GM John Ricco said the blister burst during his start last night against Fresno, and is something Wheeler dealt with last season.
Wheeler is expected to make his next start, which is scheduled for Sunday against Colorado Springs at Cashman Field in Las Vegas.
Wheeler is 0-1 with a 3.12 ERA in his first two starts of the season.
I've never heard of a player getting an MRI done on a blister, but that could have definitely contributed to his uneven start to the season. Hopefully, he is beyond this, the blister doesn't reemerge, and he can get a consistent feel for his pitches from this point forward.