Change with No Increase Yet

I was shock to know last Monday that my co-worker will resign and tomorrow will be her last day. I can't say no with the new task and responsibility. I was hoping to get the increase. When I talked to my boss yesterday, my hopes failed. He told me that the increase will start 2 months from now. I could be underpaid of my other task. That made me disappointed. I thought the increase will be automatic. Too selfish of them. But hey what can I do, it's what they think it's their best decision. As for me, it's quite unfair. I will just hold on until my salary increase start and from then on I can do my job hunting. It's better that way so I could ask more from other companies or just make my last pay as a start rate. I will cross fingers. I hope they will think it twice as to how much will my increase be.