The Whites eased through the game in a 2-0 win over Oldham at Boundary Park.
Howson set the League One leaders on their way with a powerful first-half strike which took a slight deflection before flying in from 20 yards.
Grella sealed it in injury time, condemning Oldham to a third straight home defeat.
Leeds could have gone ahead early through Bradley Johnson, who exploited poor defending but was thwarted by Oldham's debutant keeper Darryl Flahavan, a loanee at Leeds at the end of last season.
Oldham then had their best chance of the game but winger Joe Colbeck blew it with a woeful touch after he was gifted possession inside the box from a loose Howson pass.
Leeds took control following Howson's second goal of the season and most Oldham efforts were from outside the box, with midfield trio Dean Furman, Chris Taylor and Alex Marrow all unable to test visiting keeper Casper Ankergren.
The Dane's only real save came midway through the second half as he tipped Taylor's drive around the post.
Leeds wrapped up the victory on the break when Grella converted Aidan White's cross from close range.
Howson set the League One leaders on their way with a powerful first-half strike which took a slight deflection before flying in from 20 yards.
Grella sealed it in injury time, condemning Oldham to a third straight home defeat.
Leeds could have gone ahead early through Bradley Johnson, who exploited poor defending but was thwarted by Oldham's debutant keeper Darryl Flahavan, a loanee at Leeds at the end of last season.
Oldham then had their best chance of the game but winger Joe Colbeck blew it with a woeful touch after he was gifted possession inside the box from a loose Howson pass.
Leeds took control following Howson's second goal of the season and most Oldham efforts were from outside the box, with midfield trio Dean Furman, Chris Taylor and Alex Marrow all unable to test visiting keeper Casper Ankergren.
The Dane's only real save came midway through the second half as he tipped Taylor's drive around the post.
Leeds wrapped up the victory on the break when Grella converted Aidan White's cross from close range.