Mike Singletary says the Bears just got a monster in Khalil Mack ... but they didn't get the league's best defensive player -- 'cause Mike tells TMZ Sports Aaron Donald holds that honor!!
Don't get it twisted ... Singletary says his old squad ain't gonna regret giving up two first-round picks and a dumptruck full of cash for Mack, telling us he's about to wreak havoc on the NFC North.
"I know Aaron Rodgers and every quarterback in that league went, 'Wow, what the heck just happened?