Guerrero was held out of the starting lineup for the second straight night as the Blue Jays completed a three-game series with the Yankees. He is day-to-day.
Guerrero was a late scratch Wednesday and underwent an MRI during Toronto's 6-1 win.
"I was worried about it, but I found out the results and it was a little bit of relief," Guerrero said through an interpreter. "Of course I'm sad that I can't be in the lineup, try to do my best. I guess I got to take a rest or something but I'll be OK."
About two hours before Wednesday game, manager John Schneider said Guerrero's sore right knee had been "barking" for a couple of days and that he's been "grinding" through it.
"It's very hard for me not being in the lineup but thank God we're in an OK position right now," Guerrero said. "We're holding onto a playoff spot. So I'd rather take care of this now and not risk myself losing the rest of the season or a playoff."
A three-time All-Star, Guerrero is hitting .264 with 24 homers, 90 RBI and a .781 OPS in 147 games. He has played 115 games at first base after appearing in the field 128 times last season when he batted .274 with 32 homers and 97 RBI and an .891 OPS in 160 games.
The Associated Press contributed to this report.