Allardyce has been a keen admirer of his talents for some time and is looking forward to seeing him produce his best form in a claret and blue shirt.
Speaking at a Q&A event for Hammers seasonal corporate members, Big Sam said: "Stewart is a great player with a very good pedigree.
"If you look at his technical and physical stats, it puts him right up there among the top players we have at the Club.
"He's a talented boy with good energy, and will improve the team quality, particularly in possession. He doesn't give the ball away, he can hold it if he needs to and he can dribble as well.
"Stewart will be able to produce the kind of ball we need, and will have the opportunity to cross the ball, just as he did at Aston Villa.
"To pick up two players of the quality of Stewart and Andy is good business both financially and in football terms, and they will help to improve us as an attacking unit."
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