Head injuries (there’s an article at cricinfo: http://www.cricinfo.com/magazine/content… ) and career ending injuries are pretty bad but I would have to say that death is the most serious.
Several cricketers have died playing matches:
Raman Lamba died 3 days after being hit on the head (in a match between Abahani and Mohammedan) while fielding at short leg without a helmet.http://www.cricinfo.com/ci/content/playe…
George Summers of Nottinghamshire was hit on the head at Lord’s in 1870 off the bowling of John Platts. He died 4 days later.http://en.wikipedia.org/wiki/George_Summ…
Abdul Aziz, a Karachi wicket-keeper, died after being hit over the heart in the 1958-59 Quaid-e-Azam final off the bowling of Dildwar Awan. The blow apparently aggravated an unsuspected heart condition, and he died on the way to hospital. On scorecards he’s recorded as either being “absent”, “absent hurt” or “absent dead” in the second innings of the match.http://en.wikipedia.org/wiki/Abdul_Aziz_…
Ian Folley of Lancashire, playing for Whitehaven in 1993, was accidentally hit underneath the eye in a freak accident and although he continued to field during the rest of the game, he was later taken to hospital and, while under anaesthesia, suffered a heart attack and died.http://en.wikipedia.org/wiki/Ian_Folley

Tags: Case, Cricket, During, Injury, Match, Most, Serious, Which