League Two leaders Crewe have signed centre-back Christian Mbulu on a deal described by the club as "short-term".
The 23-year-old spent pre-season with the Railwaymen and bolsters a defence which has conceded 17 goals in 15 league matches.
Mbulu spent last season with Scottish Premiership side Motherwell, making six appearances, having left Millwall without playing a first-team game.
Crewe have kept just three clean sheets in 19 matches across all competitions.
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