The 27-year-old right-back made 12 appearances for Boro after joining the Teessiders from Preston in January and appeared to make a solid start.
Yet Fisher is now set to miss the start of pre-season and could be sidelined long term following a freak accident.
“We don’t actually know yet and we’re crossing our fingers,” Boro boss Neil Warnock told BBC Tees Sport last week.
“It was an accidental thing. I think he had his legs up at home and I think someone in the family just fell on his knee and just clinked it.
“We don’t know yet. He’s had three injections over the last few weeks and we should know more in about a week’s time.
“But we think he could be out for a while at the moment and we’re just crossing our fingers that it’s nothing worse.”
Sunderland man signs new deal
On the transfer front, Warnock still hopes he can make ‘four or five’ more signings before the end of the window.
One player who was linked with Boro was Sunderland midfielder Luke O’Nien, yet the 26-year-old has now signed a new three-year deal at the Stadium of Light.
After putting pen to paper, O’Nien thanked head coach Lee Johnson and sporting director Kristjaan Speakman.
“I’m delighted to sign a new contract with Sunderland,” O’Nien told the club’s website.
“I’d like to thank Lee Johnson and Kristjaan Speakman, and the owner and the staff, for putting their trust in me and enabling me to continue to play for this great club.
"Hearing their plans for the club and how they want to develop me personally was incredibly exciting and I can’t wait to get going again.”
Plymouth tickets on sale
Finally, tickets have gone on sale for Middlesbrough’s pre-season friendly at Plymouth Argyle, which will take place on Friday, 23 July (7pm kick-off).
The Plymouth match is the final friendly Boro will play during their week-long trip to Cornwall later this month.
Tickets are priced at:
Under-8s: free when accompanying an adult/over-65.
You can find more details by visiting the club’s website.