The Celtics will be without Al Horford on Wednesday night when they host the Los Angeles Lakers. The C’s big man has entered the NBA’s concussion protocol, the team announced Wednesday afternoon. Horford was struck in the head by the forearm of Atlanta forward Kent Bazemore while going for a rebound in the second quarter of Boston’s win on Monday night, but did not experience any concussion symptoms until Wednesday morning. It’s unclear how long Horford will be sidelined, but he missed nine games at the beginning of last season after suffering a concussion in practice. Given this is his second concussion in as many seasons, he could be sidelined for a some time.