The Red Sox don't have a long-term answer for that yet, but Penny will temporarily be available out of the bullpen.
When Wakefield starts Wednesday night, Penny will be there for backup in case the knuckleballer has a hiccup in his first since July 8.
If Penny is not needed on Wednesday, he will back up Tazawa on Thursday.
The Red Sox plan on having Penny pitch a simulated game on Friday if he isn't called on to pitch the two nights he is in the bullpen.
"We don't just want to turn him into a reliever," Red Sox manager Terry Francona said of Penny. "That's not in anybody's best interest. But we want to protect the club, we want to help him. That's about as far as we can go. And we communicated that to him today."
Penny has struggled mightily over his past five starts, going 0-4 with a 9.11 ERA. For the season, he is 7-8 with a 5.61 ERA in 24 starts.
The 31-year-old Penny has pitched just four times out of the bullpen in 273 career appearances.
Tazawa, meanwhile, is 2-1 with a 2.81 ERA in his three starts.
The Red Sox should get Daisuke Matsuzaka back in their rotation in a couple of weeks. The right-hander will throw three innings in a rehab start in the Gulf Coast League on Monday, marking his first start of any kind since getting shelled by the Braves on June 19.
There is clear enthusiasm from the Red Sox about the progress Matsuzaka has made since going on the disabled list.
"He is significantly stronger in his shoulder," Francona said. "He is significantly leaner in his body. And he is also significantly stronger in his body. So there's a lot of reasons to be excited."
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.