Is this the most frustrating Middlesbrough side of all time? It certainly must be up there.

A hurtful defeat to Sunderland on Monday evening highlighted a few season-long flaws of this side that continue to go unresolved. Missing good chances at one end, they’re so soft-bellied and error-ridden at the other end. Having squandered opportunities in a strong opening to expand their one-goal lead, they gave the ball away in their own half for all three Sunderland goals.

At the same time as Boro were left reflecting on another defeat that left them outside the play-off places, the January transfer window closed. A busy window in which six players arrived and six left (with the exit of Lukas Engel still pending), it’s hard to believe now that, at the start of January, there was a feeling it could be a quiet month on Teesside!

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As it happened, Boro have, as Michael Carrick put it himself, shown intent this month in their bid to get back to the Premier League. It’s a transfer window, which added to the heavy investment of the summer, it puts a lot of pressure on the Boro head coach to finally deliver.

Losing your top scorer mid-season is of course the big risk of the window. Emmanuel Latte Lath departed for a club-record fee, and it is understandable that some might question intent with that in mind. Put simply, Boro could not turn down that kind of money.

But in replacing him with a striker of Kelechi Iheanacho’s pedigree, and also adding quality in the form of Mark Travers, Ryan Giles, Morgan Whittaker and Samuel Iling-Junior, Boro’s heirarchy have made clear their promotion expectations this season.

There’s an obvious concern. The midfield lacks steel in the absence of Aidan Morris. Despite attempts to sign Glen Kamara, Will Smallbone and Tyler Morton, nothing came off for Boro on this front and they are forced to maintain faith in Dan Barlaser, while hoping Morris can avoid any more serious injuries.

In defence, there were opportunities to trade Anfernee Dijksteel for a new right-back ahead of his contract ending this summer, as well as signing a centre-back to replace Matt Clarke, who did leave for Derby County. In the end, Carrick decided he was happy to go with what he had, worried about the impact of too much disruption after transfer activity that would be considered busy in the close season, never mind mid season.

The centre-back decision is the most worrisome and least understandable. Particularly for a club that has endured the injury record it has in recent time, Boro are just two injuries away from a defensive crisis that forced them to unexpectedly sign George Edmundson in the summer.

In truth, Carrick didn’t ever really fancy Clarke despite his immense form over the past 12 months and there’s an element of being a morally good club in allowing him to go and play more, particularly after the 14 months he went through just recently, not knowing if he’d ever actually play again. But as far as self-protection is concerned, letting him go without a replacement feels a massive risk.

Regardless, on paper, that’s an incredibly impressive squad that should be achieving far more than it currently is. Monday’s defeat left Boro 14 points behind Sunderland in the Championship table. There’s no bigger indictment on the campaign for Boro, and the massive missed opportunity that not being an automatic promotion contender this term is.

But they’re still more than in the race for the top-six and have fresh faces and new quality who will need to settle fast, and providing they do, can be a huge asset to Boro in the final 16 games (and hopefully three more on top). If - and it feels a very big if right now - they can find some form between now and then, this Boro squad should fancy their chances against any side in this league when they’re at their best.

Whittaker is a significant upgrade on Isaiah Jones, with the club taking an opportunity to add a long-term target to help this season and also have the post Ben Doak world of next term in mind. Unexpectedly, they added Iling-Junior on loan from Aston Villa too - the forward, who was a regular at Juventus last season, was seen, like Doak, as just an opportunity that couldn’t be passed up on.

One interesting factor is that four loans this month takes Boro’s squad up to five - something after Carrick’s first season they’ve largely tried to avoid. In the increasing desire for more stability, it’s already looking like another big summer rebuild will be required after January. Travers’ arrival is one of circumstance after goalkeeper injuries, and it represents a big capture for Boro.

While Doak and Iling-Junior have little long-term prospect, the club hope the quality they add justify the short-term decision - which has certainly proven the case with Doak so far. Giles and Iheanacho, meanwhile, can have longer-term stays at Boro, albeit with the club having the protection of no obligation while paying low wage percentages for both for the rest of this season. In that sense, the duo are effectively on extended trials that will hopefully motivate them even more to impress in the Boro shirt.

With so many options available now, the troubles throughout the season of breaking defences down must surely be eradicated. They not only have quality in their ranks, they have variation now too.

The key for Carrick now is blending that together and creating a successful side. Top-heavy it may be, but the fact of the matter is, this squad is currently under-achieving, and the head coach has to take some blame for that. Another three goals shipped against Sunderland from giving the ball away in their own half, it's been a hindrance of this Boro side, but one that Carrick continues to insist on.

The fact is, even at the back where so many concerns remain, they have quality Championship players. In one sense they should be expected to implement the desired style better - and indeed at times they have, which only adds to the frustration. But, as Einstein’s definition of insanity taught us, Carrick may die on his sword if he doesn’t either find the improvement he keeps acknowledging is needed, or finds an alternative way.

Quite frankly, hearing opposition managers and journalists alike tell me how good or fun Boro are to watch, while then gleefully leaving with more points than they perhaps deserve, is getting tiresome. Even after just two wins in nine, Boro can go back into the top-six with their games in hand. It highlights the inconsistency of all teams outside the top four, and should provide Boro with their biggest motivation to find the next gear and put together a top-six-securing run.

This window, and those previous to this, have provided Carrick with more than enough tools. The rest is up to him and his players now. There are 16 huge games to come.