Nottingham Forest have been handed a stern warning following the appointment of Ange Postecoglou.The Australian began his tenure away to Arsenal on Saturday after replacing Nuno Espirito Santo this week.GettyForest have been warned Postecoglou is not the man to take them forward[/caption]GettyNuno was quickly replaced by the former Tottenham manager after his exit this week[/caption]Postecoglou has signed a two-year deal at the City Ground just three months after leaving former side Tottenham.His current club fell to a 3-0 defeat against the Gunners at the Emirates Stadium in a difficult start for their new manager.However, Jamie O’Hara is already doubting whether the decision to bring Postecoglou in will pay off.His arrival on Tuesday followed the departure of Nuno, who guided Forest to a seventh place finish and Europa League qualification last term.Reacting on the GameDay Phone-In, O’Hara admitted: “If they think they’re going to improve under Big Ange, I’d be amazed.“I think Nuno did a great job, a great job and I just think it’s a crazy decision to sack a manager after he just got back into Europe.“That’s the problem (not being for football reasons). But then I’d have the hump with the owner. Because the owner, alright you’ve got to back him because he’s done good things there, I understand that and fans want to back him.“This one for me, I just can’t see how it’s going to work. There’s so much around Ange Postecoglou, his interviews, the things he comes out with and I just think it’s just so many problems that come with that.“When you’re Nottingham Forest, because he’s been at Tottenham and it was a bit of a disaster in terms of his interviews and the stuff that he gave the press. It just carried it straight into them.“‘I’m not a clown. I’m not a clown mate.’ All this stuff as soon as Nottingham Forest start losing a few games.GettyArsenal inflicted a dominant 3-0 defeat on Postecoglou in his first match as Forest boss[/caption]“They’re just going to come out constantly. talkSport have done it already. They’re a shambles.”Postecoglou had been available after he was sacked by Spurs in June following their 17th place finish last term.Despite ending the club’s 17-year wait for a trophy by winning the Europa League, he was relieved of his duties at the end of the campaign.His previous Premier League job has done little to convince O’Hara, with the former Spurs midfielder adding: “I’m not having it. I can’t believe they’ve done it.“I think Nottingham Forest have shot themselves in the foot, I’ll be honest. I really do. I think it’s a really, really dodgy decision. I’m just not convinced.”Will Postecoglou succeed at Forest?Co-host Jason Cundy was more understanding of the reasoning behind the appointment, though.GettyO’Hara claims Forest have ‘shot themselves in the foot’ by appointing Postecoglou[/caption]He explained: “I know what you mean because of what Nuno did and how stable they were and everyone knew what they were doing.“They were a really good side, really good side. We’ll see how it plays out over the coming weeks, what changes he makes.“There’s not a lot to really change about Forest because defensively they’re always well organised.“It’s about him trying to imprint, keep the best parts of what Nuno was and add an attacking style of football is probably what Edu and Marinakis wanted.“They weren’t particularly impressed with the way Nuno set his teams up to play. It wasn’t aesthetically pleasing.“From my understanding they brought someone in that likes to play on the front foot. But of course that comes with its own issues.Nottingham Forest's next six matchesPremier League unless statedSwansea (A) – September 17 (Carabao Cup)Burnley (A) – September 20Real Betis (A) – September 24 (Europa League)Sunderland (H) – September 27Midtylland (H) – October 2 (Europa League)Newcastle (A) – October 5“You’ve got to get the defensive part. It’s two parts of the game, one with the ball and one without it.“We know what he’s like with the ball because he throws bodies forward. Will we see a slightly different Postecoglou? Will we see someone that has learnt from his time at Spurs?”Postecoglou suffered 22 Premier League defeats with Spurs last season on their way to finishing on just 38 points.The tally saw them end the campaign with their lowest top-flight finish since 1977, with only 14 points separating the club from the relegation zone.Postecoglou on Forest challengesHis challenge at Forest is not lost on Postecoglou, though, following their defeat to the Gunners.Speaking after the match, he told TNT Sports: “It hasn’t been an ideal prep for the players and you have to take that into account.GettyPostecoglou is ready for the ‘challenge’ after signing a two-year deal at the City Ground[/caption]“All you can ask for is effort and unfortunately, against a very good opponent, today was a bridge too far.“This is a group of players who showed last year how hard they fight for each other. Disappointment today but there’s only one way to respond.“They’re all big jobs. I haven’t taken over an easy one yet but that’s why I’m here. I love the challenge and we’ll get to where we want to.”Forest are next in action on Wednesday as they travel to Swansea in the Carabao Cup.Postecoglou will then eye his first league points at Burnley on September 20 before facing Real Betis in Europe four days later.