Joe Kinnear may have exited stage left but the impression of Newcastle United as a club in chaos was reinforced on Thursday. Willie Donachie, their reserve team manager, quit hours after he had been suspended for "disciplinary reasons" after he allegedly hit one of his players following the Under-21 side's 2-0 defeat at Sunderland on Monday night.

The 62-year-old former Manchester City and Scotland full-back had been told to stay away from the training ground while Newcastle conducted a thorough investigation into the events at the Stadium of Light.

Donachie was furious at a performance that came 48 hours after Newcastle's first team had lost 3-0 at home to Sunderland and at a time when Mike Ashley, the club's owner, has made it clear he expects youngsters to start graduating into Alan Pardew's senior side from the junior ranks.

Angry dressing-room exchanges are understood to have ensued, eventually leading to a fight during which Donachie is alleged to have struck Remie Streete, a 19-year-old defender, who had conceded a penalty.

When a complaint was made to Newcastle's hierarchy, Streete had a meeting with Pardew. Donachie was then suspended and an investigation opened, swiftly followed by his resignation.

Rated highly by many people inside St James' Park, he arrived at Newcastle in 2009 as assistant academy director before being placed in charge of the development squad – the modern name for the reserves – a year later.

Unlike Donachie, Kinnear lacked allies on Tyneside but Pardew has claimed he was "sad" to see the former Wimbledon and Newcastle manager leave St James' Park this week.

Although Newcastle's manager never wanted Kinnear appointed as the club's director of football last June, frequently found working with him frustrating and has not spoken to him since his departure, Pardew straight-batted questions about his ousting.

"I'm just sad to see anybody lose their job, as Joe has," he said. "He did a lot of good work here. But that decision was the board's decision. I was surprised that the decision was made at that time but it was not my decision. I have not spoken to him since but I'm sure I will do."

Kinnear's failure to make a single permanent signing during two transfer windows, allied to his non-replacement of Yohan Cabaye – Newcastle's best midfielder who joined Paris St Germain for £20m deal last week – has hardly strengthened Pardew's position.

"It has always been tough and I have never hidden from that," conceded the manager, who watched four security stewards wrestle an angry fan away from him as his side were sunk by Sunderland.

"I will never hide from the fans, even that fan who ran on to the pitch and ran towards me, I was not going to hide from him or run away from him. I will face it as it comes because I think it is a job where you have to be purposeful and you have to stay strong.

"It's very difficult to win trophies at this football club and therefore a lot of the time you are dealing with bad news and losing players but you have to keep your focus on what is important and the most important part of my job is winning first team games. You have to forget everything else."

Newcastle's manager – whose eighth-placed side travel to Chelsea on Saturday without the injured Cheik Tioté, Yoan Gouffran and Fabricio Coloccini – again stressed that reinforcing his squad in the summer is imperative, before suggesting he would be happy to work with a director of football again. "That decision is the board's, but I have no problem with directors of football," he said. "In the modern game, a lot of clubs have them. There is so much involved in transfers in terms of the other club, agents, the preparation for the bid and the finances now that it goes way past what we used to do 10 years ago.

I'm old enough now to have been in at a time when I was involved in the financial part of a transfer. But that part of it has gone from managers and has gone from me. That's probably how it should be."

Indeed, the suspicion is that Pardew would welcome having an accomplished deal-maker by his side. "We had a list of player targets for this window which we'll work to in the summer when recruitment will be very important," he said. "We need to make some changes, there's quite a bit of work to do. We know we've lost our best player but, hopefully, we can bring in three or four new players. Let's hope so because this club deserves it. There's money available and I'm sure we'll use it."

In the interim Pardew must experiment with Newcastle's playing system. "We've got no one like Cabaye now so we cannot be the team we were in the first half of the season," he cautioned. "We do have to change our style a little bit and find a way that can work for us."