Ferguson said: "Rio is likely to be out for around 10 days this time and we have to get to the root of this.
"He suffered a back spasm and he has done it a few times this year. He has had this problem too many times now. We have to get it sorted out.
"But we are fortunate to have such a talented boy like Jonny to call on.
"At 20 he shows such maturity. He reads the game well, he uses the ball well and his pace is good. He has all the attributes of a top player."