Blackwell said: "He's been plagued with injuries, but when we needed him at Barnsley he came in and did a great job.
"He picked up a severe hamstring injury when we won at Bristol City and although he could do with a little more time I put him on the bench on Saturday because we were short of defensive cover.
"We've now got (Marcel) Seip, (Matt) Kilgallon and (Chris) Morgan and look strong at the back. Jon is an experienced professional and it's great to have him on board."
Source: Team Talk
Source: Team Talk