Villanueva went through on-field drills on Saturday morning at the Bobby Mattick Training Center but as the day neared its conclusion, his right hand went cold.
Toronto believes the issue could be related to a circulation problem but won't know more until Villanueva undergoes a full examination.
"It usually indicates some circulation issues, so before commenting any further on it, we have to get a full workup for him," Blue Jays manager John Farrell said.
Villanueva began last season as Toronto's long reliever but eventually made his way into the starting rotation. The 28-year-old went 6-4 with a 4.04 ERA in 107 innings during his first season in Toronto.
The Dominican native is slotted to once again become a multi-inning reliever out of the bullpen. He is receiving competition for the spot from right-handers Jesse Litsch and Rick VandenHurk.