Keane and Lletget linked up well all night and it was the American midfielder who provided the Republic of Ireland striker his first goal, jinking into the box before winning a penalty.
The former Tottenham Hotspur, Liverpool and Aston Villa striker then added two more goals before Lletget scored the best of the night.
Running late into the box he collected a neat lay-off before calmly but powerfully sidefooting the ball high past the Toronto goalkeeper at his near post.
Keane and Lletget: A Hammers history lesson
Keane's time at West Ham ended in failure as he managed just two goals in nine games when the club was relegated under the hapless Avram Grant in 2011.
West Ham fans desperate to see their club avoid the drop had reluctantly accepted Keane given his links to fierce rivals Spurs.
But their fears that it was a marriage doomed to failure proved correct as the Hammers dropped into the Championship, Keane famously missing a string of gilt-edged chances in a must-win game against Manchester City.
The Upton Park faithful had high hopes for Lletget but after progressing through the club's famous academy but he never managed to make a breakthrough in the Premier League.
When he did play his one competitive first team match for the club it came in a 5-0 humbling to Nottingham Forest in the FA Cup last season.
He was never trusted enough by Sam Allardyce to handle the rigours of the English top flight after that and returned to his homeland with Galaxy in a bid to kickstart his career.
Why Lletget's goal is bitter-sweet for West Ham
His goal might leave some West Ham fans wondering what might have been had Lletget stayed at the Boleyn Ground.
But there were crumbs of comfort in that the Toronto defence clearly miss their own key man, Doneil Henry, who made the switch to West Ham back in January.
Henry was sent on loan to Blackburn Rovers where he won rave reviews for his performances in first two games. But he sustained a season-ending hamstring injury in his third appearance and is now fighting his way back fit ahead of the new campaign.