Drew Pearson is concerned for Dak Prescott and the Dallas Cowboys ... saying he believes the quarterback not playing in the preseason this year could cost the team BIGTIME.

The Hall of Fame wideout explained his stance to TMZ Sports ... saying he thinks Dak NEEDS preseason games after sitting out most of last season with an ankle injury.

"I think you need to at least go out there and start a series and get the feel of that, have that confidence that you’re ok," Drew says.

Of course, Dallas has announced Dak will NOT play in the preseason ... as they're being overly cautious with a new shoulder injury that cropped up for Prescott a few weeks ago.

Drew says he'd handle things differently with Dak if he were running the team ... telling us he believes preseason games are the key to early success in the regular season.