The 30-year-old central defender has returned to Teesside after his three-month loan spell at Bristol City came to a close.
Manager Tony Mowbray could have done with McManus' experience during his recent injury crisis and has challenged him to force his way into his team as Boro attempt to rekindle their promotion push.
Mowbray told the Evening Gazette: "I'm sure Stephen would have got a game when we were down to the bare bones a couple of weeks ago and had development players on the bench.
"As things stand, he's got competition for his place, but he's a good pro and I'm sure he will be trying his best to earn a place in the team."
McManus, whose contract expires at the end of the season, had hoped to win a long-term deal at Ashton Gate, but manager Derek McInnes' departure at the weekend put paid to that.
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