"There's a tendon that got a little flared up," said Romero, who took a no-decision and finished the season 9-14 with a 5.77 ERA. "It was more a precaution yesterday. Late in the year, you don't want to get hurt.
"When I was following through, I just felt something give on top of the knee. It was weird when I was watching the replay earlier this morning -- you could tell by the pitch and the way I jumped up, it was really painful."
Farrell said that aside from concern about Romero's knee, the Blue Jays were worried that Romero could have done damage to his arm by pitching through pain if he had remained in the game.
"The fear was that if there was any weakness in that quad area, that if he collapsed further on the mound in his delivery, does that put his arm in a different slot?" Farrell said. "So it was called a day."
Romero said he was more disappointed about his knee than anything because he takes a lot of pride in staying healthy and making all of his starts. He said he will listen to his body more during the coming offseason and, as an avid runner, he will avoid running on pavement as much as he has in the past.
Romero said that if he needs to go into the training room for treatment more often next season, he will. The lefty added that while his 2012 season was disappointing, he is looking forward to putting in the work this offseason that will allow him to regain the form that made him an American League All-Star in 2011.
Before Saturday's early exit, Romero stayed injury-free during a season in which Toronto lost three of its five starters from the Opening Day roster to injuries, including two season-ending injuries.
"It's unfortunate what happened with all the injuries," Romero said. "I know we keep tracing back to that, but a lot of these teams are healthy throughout a whole year, and if they're not healthy, they bring someone up who steps up. We had a lot of young guys that came up here, and they tried to do the best they could. It just didn't happen for us.
"The year I had didn't help much, either. I know for a fact this team is way better, and we're going to learn from this year as a group."