Tuohy sustained a torn ligament in his thumb in the 24-20 GUINNESS PRO12 win over Benetton Treviso on Sunday, and will be out for up to eight weeks after going under the knife.
The powerful lock joins Wiehahn Herbst and Luke Marshall on the sidelines, as Ulster prepare to take on European giants Toulon this weekend during the GUINNESS PRO12's four-week hiatus.
It is the 29-year-old's third long-term injury in 12 months, depriving him the chance to add to his nine Ireland caps in the Six Nations, and Doak knows what a big loss he will be to both himself and Ireland coach Joe Schmidt.
"It's unfortunate for him because he was pushing himself into contention for that Six Nations spot so he's got to get his head around that," he told the Belfast Telegraph.
"The game has become so confrontational. You have to be so robust. He did unbelievably well to come back from the broken arm, and then to be out for another while will be disappointing for him.
"He's a great ball-carrier for us and obviously his lineout work is tremendous. The way he carries and hits, you can see how valuable he is for us.
"Hopefully he can get back on the paddock for the back end of the season."
Meanwhile hooker Rob Herring is relishing the opportunity for some game-time against the French and European champions.
Herring's opportunities for the Ulstermen in the GUINNESS PRO12 have been restricted, but with club captain Rory Best expected to be rested this weekend, the 24-year-old is hoping he can seize his chance with Ulster in a perilous position in Europe.
He said: "I haven't had much game-time in the last few weeks so I'm raring to go. Realistically we are probably out of Europe, but we don't want to go there and just throw in the towel.
"We want to go and play good rugby and I'm not going there just to make up the numbers.
"I want to go there and win and as a rugby player that is the attitude you've got to have going into these kind of games."
Follow us on Facebook, join the conversation on Twitter, sign up to our YouTube channel for extensive match highlights and sign up for our newsletter for regular updates on the GUINNESS PRO12.
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland