TO the list of matters that Neil Lennon must address in the summer - primarily sourcing and signing new players, and preparing for the Champions League qualifiers - can be added one more task.

The decision by Johan Mjallby to leave his post as Lennon's assistant at Celtic after four years has created a vacancy that will need to be filled. Mjallby has been by Lennon's side since the Northern Irishman took over, on a caretaker basis initially, in March 2010.

Lennon is an autonomous figure and rarely gives the impression that he needs reassurance but in Mjallby, a former Celtic team-mate, he found an ally and a confidant. Replacing what the Swede brought to the management team will be key to future success at Celtic, assuming Lennon himself is intent on hanging around for the long term.

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It is not the first time a member of his backroom team has moved on. In the summer of 2012 first-team coach Alan Thompson was sacked after reportedly refusing to turn up for a meeting with Lennon. The club took their time before appointing a replacement a few months later, looking in-house to promote Danny McGrain from his role with Celtic's development squad.

It remains to be seen whether Lennon does similar when it comes to recruiting Mjallby's successor. McGrain is part of a close-knit management team that also includes Garry Parker, Lennon's former Leicester City team-mate, and Stevie Woods, the goalkeeping coach. Parker would seem the obvious choice to graduate from first-team coach to become Lennon's No.2, while others from the youth set-up - like Chris McCart, Stevie Frail and John Kennedy - could also be considered.

Each coach offers different skills and Celtic will look to the one whose talents most closely match what Mjallby brought to the coaching team before making their appointment.

Celtic could, of course, look to an external candidate to fill the vacancy. Lennon had a prior relationship with every member of his backroom staff and it seems hard to envisage someone from outside being brought into his circle of trust. After all, at the time of his appointment as manager four years ago he never seemed entirely at ease with the idea of a "mentor" being added to his staff in some sort of advisory role. Celtic wanted to appoint Stuart Baxter to such a post only for the Finnish Football Association to block the appointment and the matter fizzled out.

Working at a club like Celtic, with the prospect of regular silverware and Champions League football, would surely prove enticing to a number of coaches, especially those out of work. Steve Clarke, a Celtic fan, spent many successful years as an assistant before taking over at West Bromwich Albion, while Malky Mackay is available after the abrupt end to his tenure at Cardiff City.