Robbie Keane is on a roll.

The Galaxy forward scored twice Tuesday night to lead the Galaxy past CS Cartagines of Costa Rica, 2-0, in their opening game of the CONCACAF Champions League tournament.

Only three days earlier, Keane had three goals against Real Salt Lake and the hat trick earned him Major League Soccer player-of-the-week honors.

His first goal Tuesday night at StubHub Center came in the 66th minute after a pass from Landon Donovan. Keane's shot was partially deflected by goalkeeper Wardy Alfaro but the ball rolled under Alfaro's arm and into the net.

Keane's second goal came during the three minutes of second-half stoppage time when he took another pass from Donovan, got behind the Cartagines defense and drove the ball past Alfaro. Moments later, the game ended.

Keane and Donovan, the Galaxy's all-time leading scorer, "are in great rhythm together and continue to be very dangerous," Galaxy Coach Bruce Arena said.

Donovan did not enter the game until the 61st minute but "Landon gave us, obviously, an outstanding 30 minutes," Arena said.

Galaxy midfielder Robbie Rogers left the game in the 43rd minute because of a pulled left hamstring.

For the Galaxy and Cartagines, this was the opening game in the group stage of the CONCACAF tournament.

CONCACAF is a federation of club teams primarily from North America and Central America, and the first stage of the tournament is divided into eight groups of three teams each.

The Galaxy's other opponent in the group stage, Isidro Metapan of El Salvador, will open a two-game series against the Galaxy at StubHub Center on Sept. 18.

The Galaxy then will travel to Costa Rica for the second game against Cartagines, on Sept. 25, and to El Salvador to face Isidro Metapan on Oct. 24.

The Galaxy earned a berth to the tournament by winning its second consecutive MLS championship last year.

In the previous CONCACAF Champions League tournament that ended in the spring, the Galaxy prevailed in its three-team group play and advanced to the semifinals for the first time.

But the Galaxy lost in April to eventual champion Monterrey of Mexico.

james.peltz@latimes.com

Twitter: @PeltzLATimes