L.A. is Dwight Howard's final shot at returning to NBA dominance and he knows it ... so says Matt Barnes, who tells TMZ Sports it's "time to put the bullsh*t to the side" and get back to business.

Of course, it's been over 6 years since Howard's disastrous first stint with the Lakers ... and the guy's career has never been the same.

So, when we spoke with Barnes -- who played with Howard on the Magic in 2009 and 2010 -- about the reunion in L.A. ... he says it could be a new spark in Dwight's career.

"I think he realizes it's his last chance and really need to put all the bullsh*t to the side and just go out there and play basketball," Barnes says.

For his part, Howard says that's exactly what he's trying to do -- even attempting to shut down a beef with Shaquille O'Neal because he wants to focus on basketball.