Jenas worked alongside McClaren at QPR and with England, and says the under-fire Magpies head coach is an ‘outstanding football talent’.
But he revealed that the 54-year-old struggles with handling players when he steps up from being No 2 at a club to become the main man.
“I’m a big fan of Steve McClaren,” Jenas told Yahoo Sport.
“I worked with him over several years during my career and I can say with total confidence that he is an outstanding football talent, as well as being a top guy.
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“That’s why I really hoped he would be a success at Newcastle. But the sad truth is he can have no complaints if Newcastle sack him this week. In fact, I’m surprised Mike Ashley hasn’t done it already.
“I’ve worked under Steve when he has been a coach and a manager. As a coach he is brilliant – one of the best I’ve ever encountered – but his career shows that in terms of management there is something missing.
“I initially got to know Steve during his time with England. It was always going to be tough for him as England manager because he had previously been the coach while Sven-Goran Eriksson was in charge. As a player, you have a certain relationship with a coach or a number two, and it’s a completely different to the one you have with the manager.
“He was hugely respected – partly because he had been Alex Ferguson’s number two at Manchester United, but also simply because of how good his training sessions were. You were always tested mentally, it was never an easy session and there was always a clear reason for everything he wanted to do.
“But when he made the transition from coach to manager, it was almost impossible for the players to go from having a laugh and bantering with him to seeing him as England manager.
“Steve didn’t try to change things that much in terms of his relationships with the players, for instance by suddenly becoming super strict and serious.
“But that worked against him. Because as good as Steve’s training sessions were, the managers I have worked with that I consider the best are the ones that kept their distance and were a bit aloof, as opposed to being your best mate. That helped them establish their authority.
“Steve had the players’ full respect as a coach, but I’m not sure he had it as a manager.
“My next experience with Steve was at QPR, when Harry Redknapp brought him in as a coach while we were chasing promotion. Steve came in and knitted everything together, and he refreshed everybody – the players, the other coaches and the manager.
“He implemented new ideas and styles of play and tactics and got us more organised. We went from being a group of players who were pretty much winging it in the Championship, based on the fact we had a wealth of Premier League experience, to a team with a definable style of play. And it was down to Steve.
“That’s what makes Steve such a tough one to analyse. In that instance at QPR he was essentially doing all the things a manager would do. He was on the touchline and guiding our style of play.
“But it all boils down to the same outcome. When Steve is a number two, he gets the best out of everybody. As a number one, despite all of his ability, that crossover from coach to manager seems a tough one for him to make – in the English league, at least.
“Even then, at Newcastle I thought he could learn from those experiences and be the man for the job.
“But it’s just not happening. They have issues with scoring goals and with keeping them out – not a good combination. And for a club with the second biggest net spend on transfers in the whole league this season – behind only Man City – it simply isn’t good enough.
“I don’t want Steve to be sacked - not just because I like him but because I know how good he is.
“I still believe he can be a top manager, once he is able to better stamp down his authority. But there is no real case for him keeping his job.”