Trump hired Manafort in late March 2016 to lead his delegate effort with an eye on a contested national GOP convention. Toward the end of the primary calendar, in mid-April, Manafort and Rick Wiley were reportedly given control of the campaign, as campaign manager Corey Lewandowski took on a smaller role. Wiley left the campaign in late May, and on June 20, Lewandowski was fired. Manafort at that point became the clear leader of the campaign.
That was the case until mid-August, when Trump brought on Stephen K. Bannon as his campaign chief executive and elevated Kellyanne Conway to campaign manager. Then, on Aug. 19, Manafort resigned over questions about the direction of the campaign and — more important — increasing scrutiny over his ties to Ukraine's pro-Russian former leader.
You could argue that Manafort wasn't the unquestioned leader of the campaign for his entire tenure, but it's completely nonsensical to say that he played a “very limited” role for a “very limited” time. He was guiding the campaign's strategy while the primaries were happening, and he was in charge of ensuring that Trump's delegate lead resulted in him being awarded the nomination at the Republican National Convention.
There were at the very least two months when Manafort's power within the campaign was clearly without equal. And there's a pretty convincing argument to be made that he guided the campaign for about four months' time.
It's clear as day what Spicer is doing here. Manafort's ties to Russia continue to be a huge headache for the White House's efforts to play down Russia's alleged assistance in the 2016 campaign. Just a few days ago, CNN reported that Manafort was wanted for questioning in a Ukrainian corruption case.
But to argue that he wasn't a major player in the effort to elect Trump is just laughable.
Update: House Intelligence Committee Chairman Devin Nunes (R-Calif.) also seemed to play down Manafort's role in the campaign Monday. Nunes, a former member of Trump's transition team, said that “for a short time worked on the convention.”