Chacin left after three innings in Toronto's 9-7 win over the Oakland Athletics on Wednesday night. He is to have an MRI exam on Thursday.
"I feel sore. I got a little bit scared to throw the ball," Chacin said.
He was bothered by the elbow while warming up, and it didn't get better. He's never had an elbow injury before.
"I wasn't able to finish my pitches. I left the ball up," he said.
Toronto already is without right-hander A.J. Burnett, sidelined by an elbow injury.
"Something didn't look right. We'll just have to see," Toronto manager John Gibbons said.
Johnson doubled in the bottom of the fourth, and Toronto overcame a 7-6 deficit on Melhuse's run-scoring passed ball and Frank Catalanotto's sacrifice fly.
"That was a better feeling than getting any of my hits tonight," Johnson said. "I had some butterflies go through me after making that play. It felt good."
Vernon Wells boosted the lead to two runs with a sacrifice fly in the bottom half.
"It's a shame to score seven runs and not win," Oakland manager Ken Macha said.
Joe Blanton (3-4) gave up eight runs -- seven earned -- and eight hits in six innings for the A's, who had won their previous six road games.
"It was basically one swing of the bat," Blanton said of the grand slam. "The pitch was up. If I get it down, maybe I get a double play."
Catalanotto hit an RBI double in the first before Overbay's third career slam, his first since last July 23 at Cincinnati. Aaron Hill added a sacrifice fly for a 6-0 lead.
Melhuse hit a two-run homer in the second, Kotsay had a solo shot in the third and Thomas' two-run homer later in the third cut the lead to 6-5.
Oakland then went ahead in the fourth on Bengie Molina's run-scoring passed ball and Thomas' RBI single.
- Oakland 3B Eric Chavez missed his second straight game due to illness.
- Toronto RF Alex Rios made a nice sliding catch in foul territory in the second.