The Cleveland Indians star was diagnosed with Leukemia in the Spring ... and despite taking a few months off to fight the disease -- he's set to rejoin the team this weekend.

When we got Lester -- who also fought and beat a rare form of cancer back in 2006 -- out in NYC ... he told us he's happy the guy is returning to the mound.

Of course, Carrasco won't resume starting duties right away ... the Indians are expected to put him in the bullpen to pitch only an inning or two at a time for now.

But, seems Carlos is more than ready to take on that task ... in 3 minor league rehab starts this month, he pitched 3.2 innings, struck out 6 batters and only gave up 2 hits and 1 run.

By the way, he also threw 97 MPH!!!