Arsenal v Man Utd: Reality bites, but this is still a positive start - Alex Scott

You can watch Arsenal play Manchester United in the FA Cup fourth round live on BBC One and the BBC Sport website at 19:55 GMT on Friday.

That game was a pivotal point in the Gunners' season because, if they had lost it, they may as well have said goodbye to their hopes of finishing in the top four.

Instead, that result opens it all back up again.

As with all of their best performances at Emirates Stadium this season, Arsenal beat Chelsea in a really big game where they played on the front foot at a high tempo. They did the same there against Liverpool and Tottenham too.

Friday's FA Cup fourth-round tie with Manchester United is another occasion that should bring the best out of Unai Emery's team.

Seeing Arsenal's display against Chelsea was a reminder as to what they are capable of, and at the moment United are just playing so well - very attack-minded and full of energy.

It is a chance for Emery's side to make a real statement by ending Ole Gunnar Solskjaer's winning start, and of course the FA Cup is a big opportunity for Arsenal to get a trophy this season.

There is so much history between the two sides too. I think we have got one of the most exciting games of the season coming up, and it is live on the BBC.

Unbeaten run over, and reality bites

After watching them outplay Chelsea, I was left wondering why this can't be the Arsenal we see every week. Sadly they have not turned up very often recently.

A lot of the optimism from their long unbeaten run earlier in the season disappeared in December, when they had a run of three defeats in five games, winning only one.

Not losing for 22 games had added a bit of a feelgood factor after Arsene Wenger's departure in the summer and it gave the fans a lot of hope about where their team was going, and what they might achieve.

Since the run ended against Southampton, and especially with some of the results afterwards, I think reality has set in for those supporters about the club's situation.

The display against West Ham a couple of weeks ago, in particular, was one where the fans I spoke to were saying: 'Oh, we have actually got a long way to go.'

They realise there is a lot of rebuilding to do, and that it is going to take some time.

I have always thought that, which is why I would look back at Emery's first eight months in charge - and what he has achieved already - as being a very positive start to a new era for the club.

Defenders are not the only issue - it is how Arsenal defend as a team

You can see from Arsenal's running stats that Emery has implemented a new playing style and work ethic - no other Premier League team has covered as much ground this season.

The work Emery is doing on the training ground is making a difference. Making the sessions more demanding means you can play with a greater intensity during games.

Of course there is still work to be done, and the biggest improvement needed is defensively.

I don't just mean that defenders are the problem; the issue is also how Arsenal defend as a team because of the gaps being left in midfield and the lack of cover and support given to the back line at times.

But I do think they need reinforcements in that area because Hector Bellerin is out for the season. He is going to be a big miss, for the width he provides when he gets forward as well as at the back.

So the news that the club will only be making loan signings in this transfer window is disappointing - but I certainly don't think it is any reason to panic.

This is a new phase for the club - they are trying to change a lot of things, and it is going to take time. There needs to be patience from the fans, and that includes over new signings.

I understand why fans are frustrated because they want to see new arrivals to help the team kick on - but I don't see the point in rushing and bringing in players for a short-term fix.

I would look to what Liverpool have done since Jurgen Klopp took over in October 2015 as the example for Emery to follow in the transfer market.

Klopp initially worked with what he had got, figured out who could take his team forward and who he wanted to bring in, then started rebuilding.

This is his third full season in charge, so he did not turn things round overnight, but Liverpool had a long-term plan and you only have to look at the league table now to see that it has worked.

Will Arsenal's plan work? There are still issues that need sorting

Will Arsenal's plan bring the same results? We will have to wait to find out, but the departure of the club's head of recruitment Sven Mislintat shows that the rebuilding is happening at board level and with backroom staff too, and not just with the manager and players.

There are some big calls to be made throughout the club over the next few months, and they might not always prove to be popular with the fans.

And I would still question the decision not to give Aaron Ramsey a new contract, because you can see the difference he makes to the team when he plays.

I am sure finances played a big part when that call was made but Ramsey will be a big loss when he moves to Juventus - as looks likely - because he can offer this Arsenal team so much.

Mesut Ozil's situation is obviously a strange one too. While I have total confidence in Emery as a manager, I have been left confused by what is happening with Mesut, who has not featured since Boxing Day.

I would always have him in my team, because he can change a game in an instant, like he did just before Christmas against Burnley, when he set up Sead Kolasinac for Arsenal's first goal with a peach of a pass.

But you only have to look at the way Emery sets his teams up to see that he does not allow for any luxury players - everyone has to put in the same amount of hard work.

I suppose if Emery has worked with Ozil and has told him what he wants, and the player is still not doing that, then there is only so much the manager can do.

Every Arsenal fan will tell you that they want Ozil to stay at the club because they know how good he is and what he is capable of - but Emery might think he has given him enough time, and feel enough is enough.

Alex Scott was speaking to BBC Sport's Chris Bevan.