Much of the debate lies within the role of a modern full-back. Traditionally, full-backs must be solid defensively, more recently, they must be more of a threat in an attacking sense. Johnson certainly falls in the latter category.
For instance, in some modern systems of play—such as that adopted by Brendan Rodgers—the full-backs are arguably less defensive than one of the central midfielders. In that, when in possession, the full-backs are expected to provide the width in the final third, be high up the pitch, and should the move break down, it's the midfielder who needs to be able to drop into the centre-back area, allowing the centre-back to move into the wider area to do the full-back's defensive job.
Johnson is certainly renowned for his attacking qualities. He's fast, can dribble, possesses an excellent shot from distance and can provide excellent crossing from wide.
His defending qualities are less renowned and where he lacks consistency. One match he'll put in a superb defensive display, tracking back effectively, maintaining possession and looking every inch the £18 million Liverpool paid for him back in 2009.
The next match he'll be out of position frequently and easily beaten by the opposing winger.
Goals and Assists
However, despite his attacking qualities being what he is known for, Johnson has just two Premier League goals in the last two seasons, and none so far this season.
His only assist this season so far arrived in the West Ham game—a superb cross for Suarez's headed goal. Last season he provided four assists, the season before none (stats via ESPN). So five assists in two and a half seasons.
Indeed, according to WhoScored, Johnson's characteristics include being "weak" at crossing.
So is his perceived strength really a strength? Is he providing enough in the final third?
At Tottenham on Sunday, he looked defensively lacking—relying upon his centre-back too often and not tracking back as required. Nacer Chadli often exposed Johnson and Martin Skrtel was often the player who came across to recover the situation for Liverpool.
Johnson recorded a low pass completion rate of 70 percent (only Raheem Sterling had lower, with 69 percent), made just one successful tackle and no successful crosses. Liverpool as a collective were superb at White Hart Lane but Johnson was their weakest performer.
Henry Jackson at This Is Anfield writes that Johnson was "comfortably Liverpool’s weakest player on the day. He was erratic in possession, often putting his teammates under pressure with slack play. Also lacked his usual attacking thrust."
Supporters on the Red and White Kop forum also recognised Johnson's poor performance, one commenting that "Johnson still stood out as the one under performer. He wasn't bad, he just seemed to give the ball away more than most and not look to be up to full speed."
Another discusses stats and their role in valuing performance:
If you believe the WhoScored ratings he's having a storming season. I don't think he is though, really, and I lost track of how often he was beaten by his man yesterday, crosses flew in from the right. Going forward he's still a real asset. He seems patchy to me but statistically he's doing well, so who knows.
What's that about "lies, damned lies and statistics"?
@mattladson disappointing. Wouldn't be surprised, given his age and wages, if he went in the summer— John O'Sullivan (@JohnOSullivan91) December 17, 2013
Return From Injury
Indeed, Johnson's form since his return from injury has been below the standard of his pre-injury form.
The above graphic, from Squawka, requires little deep analysis—except that, aside from the dominant displays against Fulham and Norwich, Johnson's form (in all aspects of play) has been lower post-injury than pre-injury.
Johnson had one of his best performances for some time in the match where his injury, a high ankle sprain, was sustained—against Man United on September 1. Johnson's defensive score in that match is his highest this season.
The graph above is a good snapshot of the 29 year-old's play; showing consistent form for "attacking" qualities, but "possession," "defensive" and "overall" attributes lacking consistency and dipping more recently.
@mattladson Johnson not been consistent which is a shame because I like the player !— Chris James (@caymanredman) December 17, 2013
Since his return, we've seen his form fluctuate erratically from one match to the next. Perhaps he is still lacking full match fitness?
Liverpool have been linked with Barcelona's Martin Montoya, via The Liverpool Echo, and despite more recent suggestions that Montoya will remain at Camp Nou, Liverpool's interest is perhaps most indicative here.
Johnson turns 30 next May, with his contract expiring the following summer. According to reports, Liverpool face a dilemma in deciding whether to renew his deal, reported by James Pearce of the Echo.
Johnson is one of the club's highest earners, "in excess of £110,000 per week" according to Pearce. The rationale for keeping a player on such a high wage makes little sense.
Clearly, Brendan Rodgers doesn't want to lose a player of Johnson's calibre and experience. “Glen is the type of player you don’t realise what you’re missing until he is not in the team," the boss is quoted.
Indeed, Liverpool struggled in Johnson's absence—Rodgers woefully playing Kolo Toure at right-back against Southampton, before changing to three at the back in order to remove the problem.
The problem, though, is more the lack of quality replacement for Johnson in the current Liverpool squad. Maybe if that Southampton game were to be played again, Rodgers would have thrown Jon Flanagan in instead, a natural right-back.
Andre Wisdom and Martin Kelly have both been used at right-back during their young Liverpool careers, but both are more natural centre-backs and cannot provide the same attacking qualities that Johnson can.
So is the fact Liverpool missed Johnson so much due to Johnson, or the lack of replacements? There is a case for each argument.
Micah Richards is another who has been linked with Liverpool, and somebody who I previously wrote could be the long-term successor to Johnson at right-back.
Johnson is still a key player for Roy Hodgson's England and will, barring injury, be part of the squad that travel to Brazil for next summer's World Cup.
Kyle Walker is the only real competition for Johnson, and since his form has dipped at Tottenham, there are fewer calls for him to move ahead of Johnson in the England reckoning.
Chris Smalling deputised for England at right-back recently but again he lacks the attacking attributes to force his way into the first XI.
Interestingly though, according to Squawka stats, Walker is actually out-performing Johnson overall this season in all areas—except defensively! Perhaps Johnson shouldn't be too confident his place in the England side is safe.
Lack of Competition
Perhaps it's this lack of competition both domestically and internationally that creates such inconsistency in Johnson?
At Liverpool, especially with Jose Enrique out and Flanagan currently deputising at left-back, there is nobody to provide competition for Johnson. Andre Wisdom is out on loan to Derby and Martin Kelly has struggled to regain full fitness since his cruciate ligament injury.
Clearly, Johnson is a hugely important figure at Anfield and within Rodgers' system, his place is guaranteed. But should the manager sign a player to eventually replace the former Chelsea and Portsmouth player, his place will be less guaranteed—at which point, his salary will become disproportionate and only one outcome possible.
For now, Johnson's place is assured, but it could very quickly change.