Richardson signed at the Stadium of Light last summer from Manchester United for a fee of £5.5million.
"Last season was very difficult because shortly after I arrived I discovered a stress fracture in my back and the cure was rest," he said in the Sunday Mirror.
"That kept me out for four months and made me lose fitness I had built up during pre-season.
"So when I came back I had put on 10 kilos which I felt was a bit too heavy for me.
"In the last 20 minutes of games the extra weight was taking its toll and I think that might have played a part in my hamstring going.