Rosales, the veteran Argentine midfielder acquired last week in a trade with Chivas USA, was the teacher and Manneh, the speedy Gambian left-wing, was an eager pupil.
The two worked on some patterns which would allow Manneh to create space for himself and hopefully help generate more goals, something the Whitecaps desperately need as they prepare to face the Portland Timbers on Saturday night at BC Place Stadium in a Major League Soccer match that has playoff implications for both teams.
"We are working on some tactics, how to break them down," said Manneh, 19. "We need to score goals.
"Our whole team is doing a good job defensive wise. It's just attacking wise we need to figure something out and score more goals for the team."
The Whitecaps (7-5-12) hold the fifth and final playoff spot in the MLS's Western Conference with 33 points, two ahead of the Timbers (7-8-10). The Colorado Rapids are also in the mix, sitting in seventh spot with 30 points.
Vancouver has 10 regular-season games remain while Portland has nine.
Both teams are coming off losses. Vancouver dropped a 2-0 decision to the L.A. Galaxy last weekend while the Timbers were beaten 4-2 by Seattle. The Whitecaps also won 4-3 in Portland the last time the teams met in June, their first MLS win over their Cascadia rivals.
"It's a massive game for us," said Vancouver coach Carl Robinson. "We beat them in their own backyard earlier in the season.
"We have to make sure we come with the same intensity. We have to bounce back. I think it was an OK performance against L.A. but I don't think we deserved to win. We have to make sure we do deserve to win and we get the win."
Portland coach Caleb Porter said his team can't afford to squander any chance to pick up points, especially against a squad above them in the standings.
"We have to keep fighting and grinding," Porter told reporters in Portland. "This game was going to be important whether we won against Seattle or not.
"We have to get ready for Vancouver. The next nine games are going to determine whether or not we get in (the playoffs). It's probably going to come down to whether or not we get more points in the next nine games than Vancouver gets in the next 10."
The Whitecaps have just two wins in their last 10 games (2-3-5). During that span Vancouver has managed just eight goals and has been scoreless four times, including three times in the last four games.
Vancouver has speed up front with players like Manneh, Darren Mattocks and Erik Hurtado. One of the reasons the Whitecaps acquired Rosales was to insert a veteran into the lineup with a touch for delivering the ball and setting up scoring chances. Much of the playmaking has fallen to Pedro Morales — who leads the Whitecaps with nine assists — but Rosales will add some balance and poise.
Prior to joining Chivas this year, Rosales spent three MLS seasons with Seattle, recording 12 goals and 34 assists in 86 matches. Since 2011, the 33-year-old has 42 assists, the second most in MLS. He started 16 of 21 games for Chivas, collecting eight assists.
Manneh, who has struggled since suffering a pre-season back injury, has managed three goals this season but Rosales believes he has the ability to score more often.
"I'm just trying to be helpful," Rosales said. "He has the right energy and a lot of quality in his feet.
"He just needs some advice. Not just him but everybody on the field needs to be organized, in good shape for these important games."
Rosales has only been with the Whitecaps a week his influence is already being felt.
"We definitely need this kind of player on the team," said Manneh.
"All our attacking players have speed. He's a different kind of player. He's more of a playmaker and very tactical. He's technically disciplined. If we can have a playmaker like him and Pedro deliver us the balls there won't be any excuses. I am really excited to play with him."
So far Robinson has liked what he's seen from Rosales.
"He has excellent delivery, excellent distribution," said the coach. "He brings a little bit of guile we have probably been lacking the last couple of games."
At the start of the season the Whitecaps rained goals, scoring 16 in the first 10 games. Robinson said the recent goal drought can partly be attributed to teams playing better defence and his young team dealing with a long season.
"If you look at teams through the league they go through periods of time when it doesn't go their way," he said.
"It's a work in progress. We are in contention. We played some super stuff at the start of the year. We played some very good stuff recently without getting results. We're not scoring goals and it's something we need to address."
With both teams feeling the playoff pressure Saturday's game should have some edge.
"It will be a high-octane game full of passion," said Robinson. "Tackles will be flying and some good football."