I'm guessing they'll move Harrison and Riojas up in the rotation and have TBA for Sunday.
Will be very curious how they manage it though, as they rely heavily on Grubbs and Volantis to be their workhorses in the bullpen. I believe Grubbs was a starter last season so he would make sense to slide into the Sunday role but he may be too valuable in relief for them to feel comfortable doing that.
After those two, there best two on paper are Bing and Flores. But looking further into that, it seems like they are primarily midweek guys.
Grubbs and Volantis have 36.0 and 34.1 IP respectively. The next reliever who has regularly pitched weekends lately is Burns with only 12.2 IP so far.
Their 3 starters, Grubbs and Volantis make up for the lions share of their weekend IP. Taking 5-6 IP away from that with Spencer out is going to cause them to rely on those guys even more than usual, and hopefully have to rely on some low leverage guys in high leverage situations.
They want to use Volantis like we used Aschenbeck last year. He has typically been pitching on Fridays and Sundays, usually after 6-7 inning starts by Spencer and Riojas.
Would keep an eye on if Bing and Flores pitch for them tonight.