He will probably have a better idea on Friday as to the extent of the injury. He was walking slowly and with a limp in the Tigers clubhouse.
"Obviously, tomorrow will tell me a lot," Avila said. "But I've had this injury before in 2011, and I've dealt with it off and on the last three years. ... I should be all right."
The Red Sox, after scoring three runs in the inning, had Ross at third and Jacoby Ellsbury at first with one out in the second. The Tigers had the infield in when Shane Victorino hit a grounder right at second baseman Omar Infante. Ross, the Red Sox catcher, broke for the plate and was thrown out by a wide margin.
Ross tried to run over Avila and lowered his left shoulder as he went into home plate. Avila turned his right shoulder into Avila to absorb the blow and both players had their helmets fly off as they collided in front of the plate. Avila held on to the ball and Ross was out.
Avila said it was a clean play by Ross, but his left foot got stuck underneath him and it caused a hyper-extension of the knee.
"I have had a lot of big guys bowl me over," Avila said. "I would have done the same thing. He's trying to score a run and I'm trying to stop a run. It's not a dirty play. It's a good, hard play."
Avila tried to stay in the game, but he had to come out in the fourth after consulting with manager Jim Leyland and head athletic trainer Kevin Rand. Avila also took a foul ball off the mask in the fourth, but he said that was not a factor. It was all about the knee.
"Yes, the left knee was pretty rough, pretty bad," Leyland said. "I'm not really sure of the significance. The knee was pretty bad. I don't want to sound like some emergency situation, because I don't know that. But we felt like the best thing to do would be to get him out of there and hopefully be ready to play two days from now in Boston.
"I'm not really sure about that at this point."
If not, it will be up to Pena to handle the catching duties for the rest of the series, which resumes Saturday at 4:30 p.m. ET on FOX. If there's no National League Championship Series Game 7, the start time will be 8 p.m.
"If that happens, I'd feel very good about it," Pena said. "I came from Cuba for a chance like that. Hopefully, Alex will be fine. Everybody knows what kind of a beating he takes. For him to keep going out there for an extra couple of innings, that says something about his character. For him to keep battling, that was impressive."
Pena pinch-hit for Avila with two on and one out, and hit into an inning-ending double play. Pena came up in the exact same situation in the sixth and delivered an RBI single. His last at-bat came leading off the ninth inning, and he popped out to left against Red Sox closer Koji Uehara.
"Anytime a guy goes down, it's a concern," Pena said. "Everybody knows the job Alex does behind the plate, he's a big part of our ballclub. In my situation, I always have to be ready. That's a very tough position."
The Tigers signed Pena to a one-year contract last offseason to provide depth behind Avila, and he was needed. Avila twice went on the disabled list during the season, once in June with a contusion on his left forearm after getting hit by a pitch, and again in August because of concussion symptoms after getting hit by a foul tip.
Pena, who had spent the previous four seasons with the Royals, hit .297 with a .315 on-base percentage and a .397 slugging percentage. In 229 at-bats, he had 11 doubles, four home runs and 22 RBIs. The Tigers were 26-29 in his 55 starts.
Pena had not appeared in a postseason game before taking over on Thursday. Avila, who flied out in his first at-bat, is 5-for-27 with a home run in the playoffs so far, and he has also earned praise from Leyland for the way he has handled the Tigers pitching.
"I'm not happy I got in a game because he went down," Pena said. "But everybody dreams of being in the postseason, and this was a special moment for me. Today was fun for me. I just hope Alex is all right."