Warriors are set to face Saints in the Gallagher Premiership on Saturday, but Thomas will not be involved as he has now departed.
As for former Sale Sharks boss Diamond, he was appointed lead rugby consultant at Worcester in November and he will continue in that capacity for the remainder of this season before succeeding Alan Solomons as director of rugby when Solomons retires from coaching at the end of June.
Diamond will take charge of the rugby programme immediately with Solomons providing support as needed.
“Having taken the time to work with Steve in his role as a lead rugby consultant, it was the logical decision to offer him the role as director of rugby for the next two seasons,” Warriors co-owners Colin Goldring and Jason Whittingham told the club website.
“Steve is an experienced and proven director of rugby with the qualities needed to take the club to the next level and achieve our ambitions of competing at the top end of the Premiership.
“We have invested a great deal into ensuring we have all the right ingredients for the club to reach its full potential.
“We have unwavering confidence in the strength of the players and the ability of the club to achieve that goal. We will continue to do everything we can to achieve it.
“Alan has built an excellent platform for Steve to take things to the next level and achieve the full potential this club has to offer.
“Alan arrived in a consultancy role four years ago and his recruitment philosophy to develop and invest in a core of home-grown Worcester players and supplement them with X-factor talent has led to the development and retention of an exciting group of young players with more now coming through behind them in the academy.”
And Diamond is very much looking forward to taking charge at Sixways.
“We have a really exciting challenge in front of us and it’s one that I am relishing,” Diamond said.
“We have enthusiastic owners who have ambitions for the club to be established in the top six of the Gallagher Premiership and playing in the Heineken Champions Cup.
“Clearly we are not where we would like to be at the moment as a club.
"To achieve our ambitions, our results need to improve and changes need to be made. A key part of my role will be the recruitment of players and staff.
“Worcester is a hotbed of rugby with a fantastic core group of supporters.
"We want to grow that supporter base and give the city, county and region a successful and sustainable Premiership club that everyone can be proud of.”
As for Thomas's departure, Goldring and Whittingham said: “Jonathan has given his all to the club. We appreciate everything he’s done in his time at Warriors both as a player and a coach and we wish him all the very best in his future ventures."