Coming straight after Manchester City's thrilling victory over Tottenham, Manchester United's 2-1 win at Arsenal didn't even seem like the most important of match of the day, when once the fixture was generally the most important match of the season.

But for United, its significance might only be realised at a later date – for this was the first time that Jonny Evans and Chris Smalling played 90 minutes in the Premier League together at centre-back.

With Nemanja Vidic's ligament injury ruling him out for the season, Rio Ferdinand suffering frequent spells out, and both into their 30s anyway, United need a new centre-back partnership. This, surprisingly, might be it.

It's odd these two have not started a league game together before this season; United's defensive difficulties have been so severe that they have been forced to field Michael Carrick at the back in a couple of games.

Yet until now, the non-selection of this double act made sense. The only other occasion Ferguson has selected Evans and Smalling alongside each another in the league, it ended in disaster. At home to Bolton last March, Evans' tackle on Stuart Holden resulted in the American suffering an injury from which he is yet to recover. Evans was sent off, and was suspended for three games.

More importantly, they hardly seem like a natural partnership at centre-back, where a manager generally wants two players with contrasting qualities. Vidic and Ferdinand, the best duo in the world in the late 2000s, demonstrated that perfectly. The Serb was the hardman, the classic no-nonsense stopper who won everything in the air, although he could be a little clumsy. Ferdinand was less fearsome but more composed – he'd win the ball cleanly, he rarely got booked, he could distribute from the back and he had a turn of pace. Individually they were good, together they were superb.

Evans and Smalling seem too similar; they are both inexperienced and relatively lightweight, while neither appears a natural leader. They are both "modern centre-backs", a phrase used scornfully by pundits as a sympathetic synonym for "a defender who can't defend".

But they can – although they do so more effectively against certain types of strikers: "modern strikers", unsurprisingly. When Evans and Smalling were fielded together in a Champions League game at Benfica (when Sir Alex Ferguson played something of a second-string side), they were bullied by Oscar Cardozo, the 6ft 4in, bullish, powerful No9 who waltzed past Evans to thump in the opener. When used alongside each other at home to Crystal Palace in the League Cup, they conceded the winner from a set-piece – Evans lost the journeyman Palace striker Glenn Murray the way Vidic never would, and Murray nodded in to put United out.

However, when fielded against Robin van Persie, the best forward in the country this season but a player who drops deep rather than uses physical or aerial ability, they did quite well. It didn't go perfectly – Evans should have got tighter for the goal, although he was a little unfortunate the ball sneaked through his legs, past Anders Lindegaard's glove and in off a post – but he didn't leave much more than the width of a ball for Van Persie to aim at. Smalling was nearly embarrassed when Tomas Rosicky capitalised on his slip and squared to Van Persie, who contrived to miss from 10 yards, but such a slip is hardly indicative of a major flaw in his defending.

Otherwise, Van Persie had one of his quietest games all season. Those were his only attempts at goal – he averages 4.5 shots a game, the most in the league this campaign.

He usually completes 77% of passes – here he had a wretched 44% completion ratio.

Arsenal's own sloppiness contributed, of course, but Evans and Smalling played against Van Persie intelligently. They tracked at the right time, and stood off when necessary. They communicated well and passed responsibilities on when he drifted across the pitch. They didn't let Van Persie win a single aerial duel. They were disciplined too, not conceding a free-kick in the entire game.

In fact, Patrice Evra's awkward challenge on Theo Walcott in the final minute, for which he collected a yellow card and a bang on the head, was the only free-kick United conceded in their own half. And when that was launched into the box, Smalling rose highest in a packed penalty box to clear. The one final bit of danger, in the 93rd minute, saw Evans in the right place to make a block.

Scrappy, last-ditch defending – hardly the sort of thing these modern centre-backs should be doing.

But the first spell of sustained pressure United had to cope with was in the final minutes, when Arsenal sent 6ft 6in Per Mertesacker up front to be their targetman. It highlights the type of threat this centre-back combination will be vulnerable to.

For the rest of the game, Evans and Smalling did what Ferdinand and Vidic can no longer do; they defended high up the pitch, pushing Arsenal away from goal. United didn't set out anything like as deep as they are accustomed to at the Emirates – it meant their pressure on the Arsenal full-backs was continual rather than sporadic, on the break. Their approach against lumbering centre-forwards in future will be similar – control the space, force them away from the penalty box.

The key to successful defensive combinations is familiarity: Newcastle had the best defensive record in the league when they could field the same back four every week, until Steven Taylor's injury – then Liverpool took their mantle when Kenny Dalglish was able to field Daniel Agger and Martin Skrtel in 11 consecutive league games.

It's well worth Ferguson sticking with this duo – but the next match is a crunch FA Cup tie with Liverpool. They could be up against the raw pace of Craig Bellamy, the hold-up and link-up play of Dirk Kuyt, or the aerial threat of Andy Carroll. That game will teach them that a centre-back duo have to be ready for any threat.

When fit Phil Jones is another option, of course, but so far this season he seems more capable at right-back or in midfield. That might change – but until it does, with Ferdinand looking increasingly unconvincing, United's eighth centre-back pairing of the season might turn out to be their most regular combination.

Can Gerrard and Adam combine?

Liverpool's defeat at Bolton was the perfect demonstration of the need for discipline deep in midfield. With Steven Gerrard and Charlie Adam chosen in the centre of 4-4-2, Liverpool conceded their first when Mark Davies strode through the middle unmarked, then Nigel Reo-Coker repeated the trick for the second.

It is obvious that Liverpool are missing Lucas Leiva, but for now the question is whether Gerrard and Adam can play together in the centre. They do not have an understanding yet – from the four league games they have started in the same side, Liverpool have three draws and a defeat.

Norwich more than equal to Chelsea

Grant Holt identified Norwich's game-plan against Chelsea as being "to keep tight, to show them wide" on Saturday. "We know that we've got lads who can deal with crosses. That was the main aim, not to let them get through the middle," he said.

Simple but effective – Chelsea attempted 39 crosses, of which only four successfully found a team-mate. By way of contrast, Norwich matched their four successful crosses, from just 16 attempts.