"Andy can help us get across the finishing line in what is a very tight promotion battle," Blues' manager Steve Bruce said.
"I played with him at United - although it was a long time ago - and he is a top, top professional who looks after himself. That's why he is still a Premiership player.
"We have very young players at the top end of the pitch, people like Cameron Jerome, DJ Campbell and Nicklas Bendtner although they all have great potential.
"I just feel we need that extra bit of experience and quality up front."