The Celtics received some much-needed good news Sunday afternoon. After Boston blew a 28-point lead to lose to the Los Angeles Clippers on Saturday night at TD Garden, it feared Kyrie Irving also might miss an extended period of time. The star guard suffered a right knee sprain in the second quarter of Saturday’s loss. Head coach Brad Stevens said he “didn’t know anything about it” after the loss. But now we have a little more insight to just how serious (or not) the injury to Irving really is.