Argentina international Claudio Yacob arrived from Racing Club de Avellaneda in the summer as somewhat of an unknown to most of the squad, but has been praised for his no-nonsense approach as the Baggies sit sixth in the Premier League.
Foster told the club's official website: "He's amazing. I'd never heard of the bloke. I'm not disrespecting him but I didn't know anything about him. I'm not the biggest football fan in the world anyway - but he's not someone I'd come across.
"He came into training on the first day and he's exactly like he is now. He doesn't say a word, he comes in and just opens up and smashes everyone in training. He's a little live wire.
"He's that guy in front of you that, if the opposition break on us and we look like we might be a bit exposed, gets the tackles or blocks in.
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"It's great when you have someone like that - it's dependability really. He's Mr Reliable. You know you can count on him at any given time - that's how he started the season and how he's continued."
Foster has also been impressed with how quickly he and Youssouf Mulumbu have forged a central midfield partnership.
He added: "Him and Youssouf have got that central midfield nailed down. They are the same. They are so calm and know how to pick passes out.
"Youssouf will do his usual, stick his big backside out so everyone has to roll around him. It's great to watch - both are so calm under pressure - and Claudio's passing is brilliant.
"He's fantastic to have and he's got a bit of South American temperament about him - he's certainly not afraid to put his boot in. He's full-blooded, even in training.
"He's a huge addition to the team and it's amazing to get someone like him, but he's just one of a few gems we've found - people like McAuley, a free transfer from Ipswich, Billy Jones, another free transfer, they are finding these players from somewhere."