The beauty of being a defender is that you are in control of the space. In an otherwise reactive role – you respond to the opposition's moves and try to break them up – the offside rule means you can keep the striker as far away from goal as you like, providing the ball isn't behind you, of course.

Last week's column discussed the merits of Jonny Evans and Chris Smalling as a partnership following their largely impressive display against Robin van Persie at the Emirates Stadium. Sir Alex Ferguson was evidently convinced by their performance as he retained the combination despite the availability of Rio Ferdinand for last Saturday's FA Cup tie against Liverpool at Anfield. This was a completely different test for the duo; Van Persie drops deep and becomes involved in buildup play while Andy Carroll wants to challenge for crosses in the air.

Kenny Dalglish might have used Carroll to exploit United's potential aerial weakness. He had other options – in the absence of Luis Suárez, the Liverpool manager has used both Dirk Kuyt and Craig Bellamy as a lone striker. Granted, Bellamy's fitness precludes him from playing frequently and Kuyt has been having a poor season in front of goal, so stylistic concerns were far from the only reason for Carroll's selection, but his physicality made the decision easier.

However, playing Carroll with no partner played into the hands of Evans and Smalling. To oversimplify things, if a defence is up against a quick striker they defend deep to prevent him using pace in behind, if they're up against an aerial threat they defend high up to prevent him getting on the end of crosses. By playing Carroll up front alone – and hardly a great amount of forward thrust from the flanks in Stewart Downing and Maxi Rodríguez – it made Evans and Smalling's decision easy. They pushed up, stuck tight to Carroll, and he was ineffective; winning headers 40 yards from goal with no runners is not particularly useful.

Incidentally, it's worth mentioning that Carroll does have the potential to be more of an all-rounder, offering pace as well as height. Last season for Newcastle, when defenders worked out Carroll was predominantly an aerial force, he often surprised them with his quickness across the ground – he was more akin to the quick, mobile Alan Shearer in his Blackburn days than the penalty-box specialist Shearer became in his thirties. There is no obvious solution to defending against a player who offers both qualities, which makes a player like Didier Drogba positionally as well as physically difficult. Carroll's fitness problems have meant his threat on the ground has been minimal, but already he looks fitter and leaner than a couple of months ago.

Currently he's purely an aerial threat, and while quiet for long periods, he noticeably improved after Dalglish turned to the bench at the weekend. Then, Liverpool had two wingers on their natural sides, rather than the right-footed Rodríguez on the left and the left-footed Downing on the right. Downing moved to the left and Kuyt to the right and Liverpool quickly looked more suited to Carroll. Both wide men sent a couple of crosses in, with varying success.

But it was Bellamy's introduction that provided the mobility to complement Carroll's aerial threat. The Welshman often played behind Carroll, but also made runs past him, scaring the United defence with the pace he, perhaps surprisingly, retains at the age of 32. Immediately, Evans and Smalling looked more nervous, their positioning was less assured. Liverpool's dominance of possession clearly contributed to the pressure, but the United centre-backs seemed to drop deeper after Bellamy's introduction and Carroll could inch closer to his natural habitat, the penalty box.

The winner was interesting. United started off defending high up the pitch for José Reina's goal-kick (so no offside, despite Carroll and Kuyt being in an offside position) but then immediately dropped deep. Smalling needed to get into a covering position when Evans went for the header but retreated a whole 15 yards from his starting position, possibly to give himself a couple of yards head-start on Bellamy. As it happened, it was Kuyt who found himself on the end of the flick-on – primarily because of Patrice Evra's poor positioning, but helped by the fact Smalling was so deep, and therefore playing Kuyt onside. Subtly, it was the combination of height and pace that put Liverpool through.

The previous weekend, a Carroll flick-on had also resulted in a goal. Bolton were defending high and the classic big man-little man combination saw Bellamy springing on to Carroll's header to score. These two seem an ideal combination and have a more natural understanding than either has with Suárez.

Suárez's imminent returns means the Bellamy-Carroll partnership is unlikely to enjoy much more time together. Suárez is the main man at Liverpool and there's every chance that his mid-season break will see him return fresher than before, especially having played throughout July in the Copa América.

Suárez remains something of an enigma in positional terms. He can play on the left, on the right, high up the field as the main forward or deeper as a second striker. His impact upon English football has been huge, but his goal tally isn't as impressive as it should be. Liverpool are still searching for how to get him playing at his peak. Once he finds his optimum role, everyone else will have to fit around him.

But Carroll can't fit around another player, he is a No9, the static pivot around which the rest of a side should be built. Traditional centre-forwards are rarely assured of their place at big clubs these days, and the main outcome of Carroll's late flick-on might be the "late" part, and confirmation that he might be more suited to being a "plan B" than a regular starter.

Newcastle may need to switch Ba and Cissé around

Newcastle's new strike force of Demba Ba and Papiss Cissé were in action together at the Africa Cup of Nations in Equatorial Guinea this weekend. Senegal started the tournament as third favourites but after three consecutive 2-1 defeats, Ba and Cissé will return at the first opportunity.

The duo only started one match together – the defeat to their hosts in the second match – but it was interesting to see how they worked together. Ba played as the primary striker, remaining in central positions for crosses, while Cissé played around him, dropping deep and making runs to the channels. That was something of a surprise given Ba seems the more technically gifted player while Cissé is a classic No9, and Senegal's lack of success supports the theory that they might be better the other way around.

Dunne shows short corners have a long-term future

Richard Dunne's goal against Arsenal was a prime example of why sides cannot allow a two-versus-one situation to emerge when defending a corner. Robbie Keane and Stilian Petrov combined to easily go past Aaron Ramsey, and Keane crossed for his international team-mate to head in.

The need for two players closing down also shows why short corners theoretically make sense for the attacking side. It draws two defenders out of position by removing only one extra player from the penalty area and therefore lessens the defending side's numerical advantage in the box.