Despite featuring heavily last season for Liverpool, Brendan Rodgers had made it clear he could leave.
So Downing did not have to think too much about finalising the switch to Upton Park.
"I didn't have to think about it that long," Downing says. "I just said: 'OK, I will sign tomorrow.' And that was it. Done," he revealed to the Daily Telegraph.
"Brendan pulled me and said: 'I've just found out they've accepted a fee today.'
"He [Rodgers] said he couldn't promise I would play every week. That's understandable, but at my age  I need to play and there's a World Cup coming up so I want to give myself a chance to get into the England squad."
"I need to play. I could not go through that again. I'd missed three or four months when Brendan first came and the season before that I was in and out.
"I came back in the summer and with new players in the [Liverpool] team I was determined to have the attitude: 'It's up to you to try and get my shirt.' And that's how it felt until that conversation.
"It looked like I wasn't going to play [at Liverpool] and with two years left on my contract I knew they wouldn't offer me a new one.
"I didn't want to sit around because Brendan will want to keep on spending and reinforcing and wanting his own team. So I had to make a decision and Sam Allardyce gave me the opportunity."
Downing insists that he has joined an ambitious club.
"There are good ambitions here - moving to the Olympic Stadium and with Andy signing, and with the manager wanting to keep strengthening," he continued.
"I was happy to make that move. It's a great club, a club on the up and finished 10th last year and we want to finish as high as we can.
"He's made it clear to us that he wants to finish higher and I think we can do that. I liked his [Allardyce's] ambitions.
"He's not one to sit around, Sam, he wants to keep improving so I thought it was a good time to come here.
"He's quite persuasive and he's very straightforward, which is all you can ask for as a player.
"He told me to come here, come and enjoy my football and he would improve me as a player - simple things but things that you want to hear.
"I look at it as a whole: am I going to be playing regularly? What type of team is it? Am I going to fit in?
"And the stadium tops it off. All factors come into it but the biggest one for me is playing regularly I didn't have to think about it that long."
Downing, who has 34 caps, has not given up on playing for England again.
"I certainly still want to play for England," he added.
"Obviously I've not played for a long time but you never know. I just want to give myself a chance and coming here and playing well for West Ham gives me a better chance than if I had stayed at Liverpool."