The Magpies are reportedly preparing a bid for 23-year-old Tomkins but the Hammers boss feels that it would take a huge bid to prise the centre-back away.
"It's impossible," Allardyce told reporters. "I might have a problem keeping him if they bid £15 million.
The Hammers travel to Manchester United on Wednesday for their FA Cup third-round replay and, with injuries piling up, the veteran manager also voiced the need to rotate some of his regular starters to prevent further problems in future.
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"I'm going to look at the squad, the system. I might tinker with the system."
The 58-year-old was in pessimistic mood ahead of the trip to Old Trafford, bemoaning his side's lack of goals on the road so far this season.
"Away from home we haven't been very good recently," Allardyce continued. "The trouble is we don't score. It's that simple. Five goals. It's not good enough.
"You don't need to look any farther than that because that puts your defenders under too much pressure.
"You've got to put your chances away - and, away from home, we're not."