Robinson has helped the Baggies overcome a mini-slump with three wins in their last three league games.
Asked about the impact of the striker, Bilic replied: "Massive.
"He's a direct player, he's a winger, but he's a winger that is always in the box, that's what you want.
"He's a big asset for us."
Albion attempted to sign Robinson before United came in.
"He knows the league, he knows the players," added Bilic.
"He played in this league last season."