Downing, 29, made the most of his second-half chance as he gave Hammers fans a glimpse of things to come.
And Collins - who knows all about him from their time together at Villa - said: "I think we have got an absolute bargain.
"If we can get the Stewart Downing of a couple of years ago at Villa, he was tearing defences to pieces. He looked very sharp when he came on.
"Stewie is always the same. Obviously it didn't go well at Liverpool but he has done great and hopefully we can get him firing."
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