Diamond, currently on loan at Harrogate Town, has trained with the first-team squad in the early part of this week, giving Lee Johnson the chance to run the rule closely over a player he believes has a major long-term future at the club.
Diamond has been in excellent form for Harrogate, with three goals and two assists from his 11 League Two appearances so far this season.
Harrogate have made an excellent start to their campaign, currently sitting fifth in the table and just three points from the automatic promotion spots.
Sunderland made the decision to loan Diamond out on deadline day amid concern that the arrival of Leon Dajaku would limit his exposure to senior football.
Sporting Director Kristjaan Speakman said then: "Jack requires the rigours and frequency of regular first-team football and we feel this is the best course of action for his development.
“Having enjoyed a positive spell with Harrogate previously, it’s an excellent opportunity for him to demonstrate his ability more regularly and part of the process of him working his way into our starting eleven in the future.
"For Jack, at his age, it’s an important period for him and we will do everything possible to ensure it is a success for him.”
Diamond signed a three-year deal earlier this year, and his situation will be reviewed in January.
Speaking at the time, Diamond said the 'challenge' of returning to a club where he had enjoyed significant success previously was the right one.
"The move presents a good challenge for myself knowing I’ve already done a lot here and I suppose I finished on quite a high," he said.
"To come back it could be seen as a bit comfortable but I think it’s more of a challenge than anything knowing that I have to replicate what I’ve done previously.
"It was a factor in making my decision [knowing a large amount of the squad before signing] but the main thing is the football. It’s good to have friends here already but at the end of the day everyone has ambitions of making it in the game.”