I am quite certain (99.9 %) the oil pump has its' own check valve and will come with it pre-installed.

In post #8 changing the oil filter was mentioned. Was the oil also changed?

Which type oil filter housing does your car have? It should be topside on the left rear corner of the engine. I hope your mechanic uses quality german filters e..g., Hengst, Mahle, Mann or OE MBZ.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer