The thing that made Sherona so special was that the character was the diametric opposite from Trudy - Trudy was from all depictions sweet, unassuming, no trace of accent, deeply caring for Adrian, upscale dresser, a lover of poetry, etc. Sherona is tough, brassy (Brooklyn as all heck), and a scrapper. Both women were headstrong, and both cared for Adrian - Sherona in a different way, but she was what Monk needed right after the accident. Someone who was shyer and more overtly compassionate would not have gotten him to even so much as crawl out of bed after Trudy died.
One line of the opening show of Season 3 really kicked in with me - when Stottlemeyer said, "When that bomb blew, it killed my best friend's wife - and it killed him, too." Monk without Sherona will be interesting to see if they can make it work, and be believable personality-wise.