Sen. Ron Wyden (D-Ore.) underwent successful prostate cancer surgery Monday and is "recovering well," according to his office.
The absence of Wyden had caused worry for Democrats, not only because of his condition, but because he would be forced to miss several key votes.
The Senate's big vote this week will be on the ratification of the New START Treaty with Russia. A cloture vote is scheduled to take place Tuesday, but GOP senators have admitted privately that there are enough votes to approve the accord.
Wyden voted to repeal the "Don't ask, don't tell" policy this weekend.
If Wyden's vote is needed for other measures, such as a bill providing healthcare funding for 9/11 first responders, Senate leaders could work to accommodate the lawmaker.
"I expect that he will be discharged tomorrow and then he will play it by ear with regard to votes after that," Hoelzer said."I expect that if his vote is needed that the vote will most likely be scheduled for later in the week and that the senator will do whatever he can to be here."