His self-imposed sabbatical over, Landon Donovan made his first appearance of the year at the Home Depot Center on Wednesday night.

The Galaxy's all-time leading scorer entered as a substitute in the 63rd minute in the first game of the Galaxy's two-leg semifinal against the Monterrey Rayados of Mexico in the CONCACAF Champions League tournament.

But there was nothing Donovan could do as Monterrey then got the best of the Galaxy's defensive back line and scored two late goals to win, 2-1, before a crowd of 21,745.

The teams play the second game next Wednesday in Mexico, and Monterrey, which has won the tournament the last two years, is in a strong position for a three-peat. The series winner is determined by an aggregate, meaning the team that scores the most goals over both games.

"Going to Monterrey obviously is going to be very difficult," Galaxy Coach Bruce Arena said.

This is the first time the Galaxy has reached the semifinals, and it took a 1-0 lead in the 28th minute on a goal by defender A.J. DeLaGarza.

Midfielder Mike Magee used a header to bounce the ball inside the 18-yard box to DeLaGarza, who kicked the ball backward into the net.

But Monterrey's Humberto Suazo answered in the 82nd minute with a score from directly in front of the goal that evaded Galaxy goalkeeper Carlo Cudicini.

And as regulation play ended at 90 minutes, the Rayados' Aldo de Nigris slipped by defender Omar Gonzalez and poked the ball past Cudicini for Monterrey's second goal.

"A majority of that game our back line did a great job and then they were very poor in the last 10 minutes," Arena said. "Monterrey was able to exploit it."

Juninho, one of the Galaxy's starting midfielders, left the game after a collision in the 58th minute, although he was able to walk to the locker room.

Arena said he didn't yet know Juninho's condition but that his departure was not a turning point. Instead, "some of our players ran themselves out of their positions over the last 15 minutes," Arena said.

After helping the Galaxy win its second consecutive Major League Soccer title last December, Donovan -- citing mental and physical exhaustion -- took a few months off and was a late arrival this season.

CONCACAF is a federation of teams mainly from North America and Central America.

While the Galaxy and Monterrey are playing one semifinal contest, MLS' Seattle Sounders and Santos Laguna of Mexico are playing the other. Santos won the first game, 1-0.

The semifinal winners will meet in the finals in late April, and the winner of the tournament earns a berth to the FIFA Club World Cup in Morocco in December.

james.peltz@latimes.com ___