The 41-year-old forward, who is set to appear in a record sixth World Cup in June, was substituted in the 89th minute of Wednesday's game.
"I was thinking of not including him, he wasn't in good shape," Jesus said. "He was suffering from stomach pains and a general feeling of fatigue. When I substituted him, he went straight to the dressing room and threw up."
Ronaldo has scored 24 goals as Al Nassr, who are on their longest winning streak in the Saudi top-flight, sit eight points clear at the top of the table.