EIGHTEEN games, eleven goals. While parts of Middlesbrough’s team might still be stuttering following Garry Monk’s summer overhaul, no one can complain about the impact of record signing Britt Assombalonga.

The £15m man might be the most expensive player ever to have moved between two Championship clubs, but after his first-half double secured a routine win over a desperate Birmingham City side, only Sheffield United’s Leon Clarke stands above him in the goalscoring rankings in the second tier. And Clarke has scored two hat-tricks in his last three games.

Assombalonga has been a huge success since joining Boro from Nottingham Forest, with his predatory instincts and clinical finishing a key reason why the Teessiders find themselves in the play-off places despite their mixed start to the campaign.

With the 24-year-old in their side, they always look capable of scoring, and while last night’s win came against poor opposition, it still means Boro have won four of their last five matches. Last weekend’s slip-up against Leeds United was disappointing, but beat Derby County at the Riverside on Saturday, and it will hardly have been disastrous.

Assombalonga’s double last night came courtesy of a close-range header after Birmingham goalkeeper Tomasz Kuszczak fumbled Stewart Downing’s shot, and a crisp low finish after Fabio Da Silva unpicked the Blues defence with a fine ball over the top.

Had he added another goal in the second half, he would have become only the fourth Boro player to have scored a hat-trick at the Riverside. On the evidence of his performances so far, it might only be a matter of time before he claims that honour.

His goals capped a display that was workmanlike rather than wonderful, but as Aitor Karanka can attest, routine victories built on a strong defensive platform can often be the key to winning promotion. Boro’s defence was rarely extended last night, with both Daniel Ayala and Ben Gibson quietly effective.

In truth, this was a game any promotion hopeful should always have been winning. Birmingham travelled to Teesside with the worst away record in the Championship, having picked up just one point from their eight previous matches away from St Andrew’s. Confidence under Blues boss Steve Cotterill is all but non-existent, so with that in mind, it was imperative Boro made a strong start to further demoralise their opponents. A breakthrough after just ten minutes was just what they required.

There was an element of fortune to the home side’s opener, with Kuszczak fumbling Downing’s 20-yard strike after an injury to visiting captain David Davis created a pocket of space for the former England winger to break into.

Kuszczak’s attempted parry sent the ball spiralling towards the edge of the six-yard box, and the onrushing Assombalonga reacted quickest, heading home from close range. It took the striker into double figures for the season, and his 11th goal would quickly follow. Other parts of the Boro team might be misfiring, but the Teessiders’ record signing is more than justifying his price tag.

Boro’s attacking problems have tended to stem from what has been going on around Assombalonga, but this was a night when things clicked smoothly into gear. The recalled Marvin Johnson provided some welcome width and energy on the left-hand side and Martin Braithwaite pulled the attacking strings as he dropped off into the vast expense of space Birmingham were leaving between their defence and midfield.

The Dane narrowly failed to find Johnson midway through the first half as he tried to pull the ball back after breaking clear of the Blues defence, and his intelligent movement troubled the Birmingham back four all night.

Boro’s backline were barely extended at all for the majority of the opening period, with Ayala clattering into a succession of aerial challenges, but as their opponents gradually grew into the game towards the end of the first half, so the hosts suffered a couple of scares.

Ayala slid in to produce an excellent block as former Boro full-back Jonathan Grounds shot from the edge of the area shortly before the half-hour mark, before Darren Randolph produced a fine point-blank save to prevent Che Adams from equalising moments later.

Sam Gallagher’s headed knock-down teed up Adams inside the area, but Randolph raced from his line to keep out the striker’s shot. When the visitors recycled the ball from the left-hand side, Cheikh N’Doye headed wastefully wide from eight yards out.

It was a miss that robbed the visitors of their only realistic route back into the game, as just seven minutes later, Boro doubled their lead.

Having been recalled at left-back in place of an out-of-sorts George Friend, Fabio lofted a superb ball over the top of the Birmingham defence to find an unmarked Assombalonga in the area. Boro’s top scorer hardly needed a second invitation, and after steadying himself, he rifled a clinical finish past Kuszczak.

He came close to claiming a hat-trick on the hour mark, only to direct a back-post header straight at Kuszczak after Cyrus Christie delivered a hanging cross from the right, and his performances over the remainder of the season are likely to be crucial to Boro’s promotion prospects.

A third Boro goal looked imminent for the majority of the second half, and Jonny Howson almost provided it when he whistled a shot narrowly over the crossbar. On a more negative note for the former Leeds midfielder though, the booking he picked up at the start of the second half was his fifth of the season, meaning he will have to sit out Saturday’s home game with Derby.

That probably explained why Monk was keen to give Adam Forshaw some game time in the final 25 minutes, with the substitute’s introduction coming moments after Boro hit the woodwork.

Fabio’s forward runs were a feature of the home side’s attacking play all evening, and after cutting in from the left-hand side, the Brazilian crashed a rising effort against the underside of the crossbar with Kuzsczak beaten.

Birmingham’s second-half efforts were desultory, with Jota proving especially disappointing. Earlier this summer, Boro were bitterly disappointed to have missed out on the Spaniard, who cost £6m when he left Brentford on transfer-deadline day. On this evidence, they had a lucky escape when he chose St Andrew’s ahead of the North-East.