The Baggies goalkeeper was mesmerised by the Belgian's impact as he scored a second-half hat-trick in the incredible 5-5 draw with champions Manchester United yesterday.
And he reckons it will be the last time Lukaku runs out in Albion colours.
"There's no way he's coming back here next season," said Foster.
"Chelsea would be fools to let him come back with the way their forward line is misfiring, he's exactly what they need at the minute.
"He has come on and changed it. Fair play to him.
"He was unplayable. As soon as he came on you could see that they were physically scared of him. He dominated everything that came his way. It was a fitting send-off for him.
"He's done so much for us and I'm really happy for him because he's such a nice lad."