Rob Gronkowski rehabs in Foxboro

Tight end Rob Gronkowski, who is recovering from back surgery, has returned to the New England Patriots' facility in Foxborough, Mass., where he is doing his rehab work under the guidance of the team's trainers.

Gronkowski is concentrating mainly on stretching and improving his core strength.

A person who has seen Gronkowski at the facility tells ESPN that Gronkowski looks good and seems to be progressing, but nobody yet knows whether he'll be ready for the Patriots' opener against the Buffalo Bills.

Gronkowski, who was at a book signing in Boston on Thursday for "Growing up Gronk," wouldn't comment when asked about the condition of his back. He did, however, comment on his mental outlook heading into this season.

"I can tell you this, I'm in great spirits and I'm enjoying life a lot more. I can tell you that right there," he said.

Gronkowski had the surgery on his back last month and Dr. Robert Watkins, who performed the procedure, said the "timing of his return to football will depend on his progression through the rehabilitation program."

He also had a fourth surgery on his left forearm in May, where doctors put new plates in to repair the previous break.

He is expected to open training camp on the physically unable to perform list, a league source told ESPN NFL Insider Adam Schefter last month.

Information from ESPN NFL Insider Adam Schefter and ESPNBoston.com's Mike Reiss was used in this report.