The Angels announced the temporary number change on Twitter. La Stella wears No. 9 and Trout, a two-time American League MVP, has worn No. 27 since debuting in 2011.
"A tribute to Tyler Skaggs at the #AllStarGame," the team wrote.
Skaggs pitched for the Angels since 2014. He and Trout were both drafted by the Angels in 2009, although Skaggs was traded to the Arizona Diamondbacks a year later.
Nationals pitcher Patrick Corbin wore No. 45 instead of No. 46 last week in his first start since Skagg's death. Corbin struck out seven Marlins hitters in seven innings of one-run ball in the Nationals' 3-2 victory.
Corbin and Skaggs were teammates in the Angels' Minor League system and with the Diamondbacks from 2012 to 2013.
La Stella is enjoying a career year with the Angels. He was hitting .300 with 16 home runs and 44 RBIs in 78 games before fracturing the tibia in his right leg. La Stella is not playing in Tuesday's All-Star Game.
Trout is hitting .301 with 28 home runs and 67 RBIs in 87 games this season. The eight-time All-Star was the game's leading vote-getter.
The MLB All-Star Game is streaming live on Fox and ESPN Radio.