Capela averaged 16.6 points and 12.7 rebounds per game for the Rockets last season.
Horford is planning to leave the Celtics and sign a four-year contract with a different team, according to a report from the Boston Herald.
The Celtics have their eye on Vanderbilt guard Darius Garland with the No. 4 pick.
Al Horford and the Celtics want to work out a new deal in July.
The Nets are reportedly the favorites to sign Irving while he has also been linked to the Knicks and Lakers.
Horford, 33, has one year remaining on a four-year, $113 million deal and a player option he may decline for the 2019-20 season.
Tatum said in January when there was speculation he could be traded for Davis: "Yeah, I'd trade me too for Anthony Davis."
The six-time All-Star told the Pelicans in late January that he didn't plan to re-sign with the franchise and he wanted to be moved.
While some believe Russell and Irving could coexist in a backcourt together, it doesn't appear they'll get the chance.
The Celtics' confidence they will be able to re-sign Kyrie Irving has "eroded" but they hope a deal for Anthony Davis could get him to stay.