Shaun Hobson will be sidelined until at least next month after suffering a broken foot in midweek.
The Latics defender had to be helped off during the second half of their opening National League Cup game against Stoke City Under 21s on Tuesday night, which Oldham won 2-1.
The defender was sent for an X-ray, with manager Mellon receiving the disappointing news that Hobson has been ruled out for at least four weeks.
Depending on the nature of the break, such injuries take between four and six weeks to heal, more commonly the latter.
It is a cruel blow for former Southend stopper Hobson, who was performing well in the back three having recently worked his way back into the starting XI.
Latics, who return to National League action with the visit of Solihull Moors tomorrow (Saturday, kick-off 3pm), have options on the right side of defence, most notably in Reagan Ogle and Will Sutton.
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