The on-loan Italian midfielder was booked early in the Championship match on Tuesday and was handed his marching orders after collecting his second in first-half stoppage time after referee Mike Dean adjudged him to have taken a tumble in the box.
Forestieri had helped Watford go ahead after 29 minutes at The Valley, with his corner nodded home by 18-year-old Tommie Hoban in what was only his third first-team appearance for the Hornets.
The defender took the credit despite a deflection and saw Charlton draw level six minutes later through Ricardo Fuller’s thumping strike.
The game’s complexion changed when Forestieri was given his marching orders and, while spending most of the second half on the backfoot, another loanee, Almen Abdi, curled home an expertly-struck free-kick to hand Watford all three points.
Zola named the same side that defeated Huddersfield 3-2 at the weekend for the npower Championship clash, while the hosts made one enforced change as Fuller came in for the injured Johnnie Jackson.
The match began sloppily as the rain fell in south London, with Daniel Pudil and Fuller wasting the only two chances of the opening exchanges.
Troy Deeney stung the palms of Charlton goalkeeper Ben Hamer as the visitors looked for the opener, before Salim Kerkar saw a free-kick deflected just wide of Manuel Almunia’s left-hand post.
It was the second attempt at the set piece as Forestieri was booked for encroachment at the first, before the Italian attacker then helped the game burst into life.
The 22-year-old – one of six loan players in the starting line-up – swung in a corner and Hoban nodded home his first senior goal via a deflection.
Watford’s lead lasted only six minutes, though, as Dale Stephens threaded a side-splitting ball to Fuller, who buried it in the bottom right-hand corner.
The teams went in level at the break but only in terms of the scoreline as Forestieri received a second booking in stoppage time for what referee Dean deemed to be simulation.
Incensed Watford players surrounded the official and Pudil was booked as the players trudged off for half-time.
Charlton’s numerical advantage saw them dominate the majority of the second half, with Bradley Wright-Phillips denied by Almunia shortly after the restart.
Wright-Phillips curled over under pressure from Neuton as Charlton continued to press, before Almunia saved another effort from the striker with his feet.
Chris Powell’s side may have bossed the second period but fell behind in the 70th minute when Abdi expertly curled home a 25-yard free-kick.
Troy Deeney then saw an attempt to double the Hornets’ lead well saved as the game became an increasingly tense affair.
Charlton camped themselves in the Watford half as they pushed for a late leveller but could not find a way through, with Stephens coming closest from 20 yards.