At some stage in the second half, the cameras will pan to the two technical areas. The commentators will wonder idly if the respective managers can, without touching the ball, exert an impact. It is part of the routine of football broadcasting.
It is also particularly pertinent in the meeting of the manager Manchester United could have appointed and the one they did select.
The "Snubbed One", Jose Mourinho, is a master of substitutions. His decisive nature, analytical brain and deep squad equip him to alter matches.
There is ample evidence of his catalytic powers: look at the wins against Norwich City, Cardiff City, Manchester City, Southampton (home and away) and Derby County this season, all fashioned with considerable input from replacements.
Then there is the "Chosen One", David Moyes. In many facets of leading a superpower, the burden of proof is on the United manager. This is another. Moyes has the supersized task of proving he merits comparison with his immediate opponent today as well as his predecessor.
Sir Alex Ferguson’s first Uefa Champions League title victory was sealed in memorable fashion by two scoring substitutes, Teddy Sheringham and Ole Gunnar Solskjaer. His second, though less spectacular, came after United’s last three penalties in the 2008 shoot-out against Chelsea were converted by Nani, Anderson and Ryan Giggs, replacements all.
Substitutes were indelibly associated with many of Ferguson’s finest moments. His reign was defined by comebacks and late goals.
Thus far, it is harder to say the same of Moyes. His team have been criticised for losing their way in the latter stages of games, for appearing at their most insipid when Ferguson's side were at their most inspired.
Moyes’s United have not scored in the last 10 minutes of a league game since September. They have had only three goals from mid-match introductions and one, Ashley Young’s against West Ham United, did not alter the outcome.
It leaves him with a mere two catalytic changes: Danny Welbeck’s December appearance at Norwich and Javier Hernandez’s October cameo against Stoke City.
Moyes’s troubles reflect particularly on the stagnating super-sub. Hernandez was Ferguson’s favourite rescue act, becoming the Ole Gunnar Solskjaer of his generation, yet since the managerial switch his decisive interventions have become altogether rarer.
Only Stoke, in both the league and the English League Cup, have recent reminders of his prowess as the 12th man.
Only Adnan Januzaj has consistently posed a threat as a substitute and he deserves to start. Other ploys have been ineffective.
Moyes went through a phase of bringing on Ryan Giggs, hoping the veteran’s experience and calmness would help United see games out. Instead, they conceded late equalisers to both Southampton and Cardiff.
Left field moves, such as turning to the forgotten men Wilfried Zaha and Anderson against Newcastle United, have looked more bizarre than brilliant.
The many wingers – Young, Nani, Antonio Valencia – have not been influential enough replacements and an attempt to keep premier players in reserve failed; Robin van Persie and Moyes’s biggest buy Marouane Fellaini were brought on against West Bromwich Albion, but United still had an unexpected home defeat.
Most disastrous was the decision that backfired to send United spiralling out of the FA Cup this month. It was not entirely Moyes’s fault – Fabio da Silva only came on because Rio Ferdinand was injured – but the Brazilian defender’s red card gave Swansea City the impetus to score a winner. He proved the wrong sort of impact substitute.
Perhaps Moyes’s inability to affect games is simply a reflection of the season-long failings of United’s squad players and highlights a lack of strength in depth that, with Wayne Rooney and Robin van Persie injured, could be more apparent today.
Yet seven fringe figures will be on the bench again and Moyes may turn to them, looking for a saviour. Indeed, given United’s mediocre record against the top nine teams – one win in nine league games – it is probable he will.
In such moments, managers can shape their future and their reputation, let alone one game. Ferguson and Mourinho have done it before. Can Moyes now?
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