Manager Alan Pardew has confirmed that Lambert will be handed the coveted number seven shirt, previously worn by Southampton legend Matt Le Tissier.
The Saints boss says his new striker will definitely start against Northampton Town in Tuesday's Carling Cup first round game.
Pardew told BBC Radio Solent: "He's someone we tried to get before Saturday's game. We'd like to thank Bristol Rovers for their help in making it happen.
"I feel he is the player that can get us out of this division and he starts on Tuesday.
"For the last two years, he has been prolific in League One and we want him to continue that. He's what I would call a marquee signing."
Lambert started his career at Blackpool and has previously played for Macclesfield Town, Stockport County and Rochdale.
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