The Ecuador striker would appear to be on the cusp of a move to the Hammers from Mexican side Pachuca following an impressive World Cup campaign from the 25-year-old.
Valencia scored three goals for his country in Brazil, but had earned attention with a header in a 2-2 draw in a friendly with England just before the tournament began.
Prior to that, he had bagged 18 goals in 23 outings for his club, and it is this kind of form that Downing hopes he brings to the Boleyn Ground along with Mauro Zarate, the striker who joined the club in May.
"For large parts of last season we played without a striker. We can't rely on Andy all season," said the winger regarding Carroll, who scored just twice last term in a campaign marred by fitness problems.
"He was the big signing last season, but Andy struggled with injuries so hopefully the new signings will take the pressure off him a little bit. And from what I've seen of Valencia against England, he looks a very good player.
"He can nick a goal and it is competition for places. It's what we've needed as we didn't have that last season. It will help the manager that he has options and he can bring other people in who can perform straight away."