Mike de Geus -- who worked for the Secret Service during the Trump years, and who used to guard the former President -- tells TMZ there were a lot of problems he saw with the way the Secret Service handled things Saturday, including several glaring issues they missed.

He points out that there are very simple things the agency could've done to block off the clear line of sight Thomas Matthew Crooks had from a building outside the perimeter ... even if that technically wasn't within the Secret Service's ring of protection.

Mike says they should've seen the building Crooks perched himself on had a direct line to Trump -- and he suggests there are simple things they could've done ahead of time to block that clear path ... including rolling out vehicles, or even turning the stage.

While he says they missed opportunities to protect Trump, he does note they did the right thing by getting him out of there relatively slowly.

That might sound counterproductive, but Mike points out the Secret Service had to make sure Trump wasn't bleeding beyond just his ear grazing, so he gives them credit there.

Now, on the issue of the agents themselves -- something else that's come under a microscope -- Mike does say he feels some of the agents might've not been the best options to protect a guy as big as Trump. The much-smaller female agent comes to mind here.

He believes the agents should be physically capable of carrying whoever they're tasked with shielding -- and, at least to the eye, it's unclear if all the agents on his Saturday detail were right for that specific scenario.

Big picture ... Mike thinks the Secret Service has slipped a bit in quality -- and while he can't pin down why that might be, he thinks this incident is a come-to-Jesus moment.