Immediate supervisor quits a month after I am hired, have to now do my duties and theirs. Can I ask for a raise?

I was hired about 45 days ago at a non-profit as an associate. My immediate manager was at the organization for over 5 years and was training me/making sure I was learning the ropes. A month in, she quit effectively immediately. As a result, I was thrown into the fire much sooner and have to do her work as well as my work. They have already made it clear that they do not intend to hire a replacement. My job requires a 90 day probationary period anyways, but would it be justified/appropriate to ask for a raise following the end of this probationary period as I am now effectively doing an associate and a manager's work myself?

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