"It was very frustrating I had put so much hard work in over pre-season and then I just felt my hamstring go against Watford," Rivers told The Pink'un.
"It's also been frustrating because the lads have done so well and you want to be a part of that, instead I have been sitting in the physio room.
"But it is important that you don't rush these things," he added. "Dwight Yorke did exactly the same and tried to come back after two weeks and now he's out again.
"I feel great at the moment, I trained with the kids yesterday and they are very quick and very physical, much the same as a Division One game and I found it fine."
"My basic fitness is still fine and hopefully with a bit more work I will be ready for the Millwall game."