A man in his 20s was stabbed Sunday morning in East Boston and had serious injuries after being robbed of his watch, police said. The stabbing occurred around 7:10 a.m. at 161 Cottage St., said Boston police Officer James Kenneally. Kenneally said the man, whom he did not identify, was taken to Boston Medical Center with life-threatening injuries. The victim’s condition could not immediately be determined. Officers later arrested Yertan Tatibayev, 23, of Boston and charged him with assault and battery with a dangerous weapon and armed robbery.