Forced to bring in long-term loan signings Steven Caulker and Jordan Spence to cover problems at the back this campaign, Millen has been hindered by a series of injuries to key defenders.
He admits a foray into the transfer market this summer will be needed if City are to avoid similar problems in the future.
"I will be trying to strengthen the defence. Even allowing for all the injuries, there are positions that require strengthening and people know that," said Millen, who has put a left-back and a centre-half at the top of his wanted list.
"We have good competition throughout the side but the only place I don't have that at the moment is in the back four. We are down to the bare bones in defence and I cannot see that situation improving until the summer.
"The summer seems a long way off at the moment but we are already looking at players we would like to bring in and that situation has been ongoing for quite a while now."
Source: PA
Source: PA