Both players arrived at Upton Park in the close-season - from Metz and Pachuca respectively - as Allardyce looks to produce a more attacking brand of football, and the pair have taken to life in England with ease.
Valencia's superb header at Turf Moor on Saturday was his second of the season following a stunning strike at Hull City, while Sakho has been even more impressive, scoring on each of his six starts in all competitions.
And Allardyce believes their impact has given the whole club a boost.
"Our two centre-forwards that we've bought this year paid massive dividends with the quality of the headers off brilliant crosses," he said.
"The lads have confidence that if they give them the right supply they're going to score.
"He [Sakho] is a bargain. This lad is being very, very exciting and he excites the rest of the players. All the players who were here last season had some depressing times and they've all been lifted by the new players and their performances have increased.
"We just hope it continues. It's too early yet to say what we're going to do but things are looking very good
"If we get to 12, 14 [games] and into December and are still getting results and maintaining a top-half position then we can start to think about retaining that for the rest of the season.
"We're doing exceptionally well in our results and we're doing exceptionally well in that we've still got players injured and nearly ready to come back into the squad."
Allardyce also provided updates on the fitness of James Tomkins and Mauro Zarate, who missed the trip to Burnley and were replaced by James Collins and Mark Noble respectively.
"Tomkins picked up an injury in training on Thursday and Zarate picked up an injury a week last Friday in a behind-closed-doors game, sadly," he said.
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