Boro boss Tony Mowbray has been looking to bolster his striking options, and now he has managed to do that with the signing of 20-year-old Nimely.
"Alex is a lad we've been watching for a while," Mowbray said of the Liberian-born England youth international.
"He picked up an injury in the summer but has been back in training for a few weeks now.
"I've spoken to (City coach) David Platt and Brian Marwood (City's football administrator) about him and they've allowed him to come here on loan.
"Having him here will give us some depth on the bench and some competition up front.
"I'm sure he'll be looking to impress and force his way into the team."