Midfielder Panutche Camara is facing months on the sidelines having undergone another operation on his groin injury last week, however, the Guinea-Bissau international should be back by the end of the season.
The 25-year-old has made only one start and three sub appearances for the Blues, scoring once, since signing from Plymouth on deadline day due to the injury, which he first suffered last season while with the Pilgrims.
"Panutche had surgery last week on his adductor area,” McKenna revealed. "The specialists felt there was more surgery required to a slightly different part of his adductor region.
"It’s a really complicated area to have injuries, which I know from personal experience.
"There’s a good level of confidence that this surgery will hopefully be a longer-term resolution and he’ll be able to get back to his full health and full capabilities, but that’s going to be a decent period of time away.
"He’s got a period now of rehabilitation and recovery and has got to go through that process now.”
Quizzed on whether a timescale had been pencilled in for the Camara's return, McKenna added: "To be honest, with the injuries that he’s had in that area and he’s had I think three surgeries now, one at Plymouth and two with us around that area, it’s hard to put an exact timescale.
"It’s going to be a few months, not a few weeks, but we he’ll be back playing football again this season.”