"He's had some back issues in the past," said Red Sox general manager Theo Epstein. "It was bothering him a little bit during Spring Training and then it flared up a lot last week, most acutely in the last 48 hours. We just have to let it calm down and get him back to 100 percent."
With Riske out of the mix, the timing of Julian Tavarez coming back from his 10-day suspension on Thursday is perfect.
"It will be good," said Red Sox manager Terry Francona. "It hasn't cost us, but if you go a long time with a pitcher short, it will get in the way. [Tavarez is] a guy that we think is going to throw a lot of innings -- and not necessarily early in the game. He's a nice bridge at times, if you have a pitcher [out] after five innings and you need somebody, he's a good guy to go to. He's going to throw strikes. He'll get righties out. The game is not going to speed up on him. There's a lot of good positives."
Nixon timetable: Francona thinks that right fielder Trot Nixon, who sustained a mild left groin strain on Tuesday, will be out anywhere from four to six days. A disabled list stint does not seem likely at this point for Nixon.
"Yeah, he's a little sore today," said Francona. "I was hoping it would be a day or two. I think it's more likely to be of the four-, five-, six-day variety. It's not killing him. He can feel it when he does a certain move."
Wily Mo Pena and Dustan Mohr will share the playing time in right field until Nixon returns.
Pena's progression: Pena had the misfortune of having a ball deflect off his glove and into the bullpen for a two-run homer in Tuesday's home opener. But Francona noted that it was a difficult play.
"I went back and looked at that play about 10 times," Francona said. "That was anything but routine. He got a couple of feet off the air and his back bumped the wall and again, I think with more familiarity, he will make that play. But I don't think that's routine either."
Francona added that the club will put in some extra time with Pena before games so he can familiarize himself with Fenway's spacious right field.
Wallace boosts spirits: One of the highlights of Tuesday's opener was the presence of pitching coach Dave Wallace, who is on extended medical leave as he battles significant hip woes.
Wallace was announced during pregame introductions.
"I think Wally is hoping [to come to Fenway] at least once a week," Francona said. "That was a big deal for him yesterday. Just to drive in was probably almost more than he's done. It took him 45 minutes or so. I was so glad he showed up. He wanted to come in today [instead] because of all the hoopla [Tuesday], but we kind of talked him into coming and they introduced him. ... I was really glad he did that. That was a nice feeling. I thought he looked great. He's been through a lot."
How often does Wallace confer with interim pitching coach Al Nipper?
"They've talked a couple of times," said Francona. "Again, you're trying to be respectful. He's been in the hospital. I actually think the games are hard for him to watch, and I understand that. But he knows how welcome he is. Because of our relationship, he can come in any time. It's not a forced issue. There's a lot of comfort there."
Coming up: Right-hander Matt Clement (1-0, 5.14 ERA) will face Toronto left-hander Ted Lilly in Thursday's 7:05 p.m. ET finale of this three-game series.