Downes, 23, will be out for around nine months after rupturing an anterior cruciate ligament in the goalless home draw against Darlington over Easter.
The Australia international is out of contract in the summer but was offered a new deal prior to his injury.
Richardson said: "The offer has been made and we will talk about it during the week.
"For me he is the best young central defender around at this level. He was in awesome form before he was injured."