There was interest in bringing the right wing-back back to the club, but the 22-year-old instead joined Sunday’s opponents Tottenham, although he has not featured in the last two matchday squads.
Cooper could have stayed in the hunt for Spence, but said the club opted to strike when Williams became available, sealing him for £17million from Liverpool.
“What we did was look at all the options at every single position and it was important to get some early ones done,” he said.
“Neco was in high demand, we had a lot of opportunities and we were in the mix for him because he could have been available from Liverpool. Once we met and both wanted to do it, it was then we felt we needed to do it – it was too good an opportunity.
“We’ve done it with other players where we have signed them because the deals are there to be done and we couldn’t take the risk of waiting for others.
“It was topical with Djed. He was part of last season’s success but it was a case of not losing out on Neco rather than waiting for one or two others.”
With Spence at least third choice for the right wing-back position at Spurs – and possibly also behind Lucas Moura and Ivan Perisic in the pecking order – chances of regular football at this stage look limited.
Cooper did not want to talk about Spence’s predicament this season, but was complimentary about his time at the City Ground.
“I can only talk about Djed from last season,” he said.
“He was obviously excellent for us. We wish him well about the two games when we face them. Current circumstances I am not too sure of and with respect, it’s nothing to do with us.
“He was good to work with, he’s a great guy and we wish his career well.
“Both him and Neco are attacking players, I don’t want to compare them as it’s not fair and Djed is a Spurs player now. That is not my domain.
“We are really happy with Neco. He has had an excellent start to playing regular Premier League football. He got that regular rhythm at Fulham last season which I think was important.
“For him to carry on, he has had a really good start and done some really good things and I know there are things he is desperate to get better at.”