Watts struck first for the League One side at Home Park, turning home a cross from Conor Grant with just three minutes on the clock.
His third goal of the campaign set the tone for what was an entertaining encounter, which looked to be about to finish at 3-3 before Joe Edwards’ stoppage time winner for the hosts.
Watts played the full game for Plymouth, and has established himself in the side with 35 appearances in all competitions so far this term.
It was a memorable day for another United loanee, too, as Lewis Cass celebrated his birthday by helping Hartlepool United earn a 1-0 win over Barnet in the National League.
Cass lined up alongside former Magpie Jamie Sterry at Victoria Park, and turned in a solid display to keep out the Bees on the day he turned 21.
Luke Armstrong netted the winner for Pools as they maintained their push for promotion, with Cass playing the full 90 minutes on his 22nd outing of the season.
It wasn’t quite as productive a weekend for some of the Magpies’ other loan players, however, with Freddie Woodman beaten three times as Swansea City slipped to a 3-1 Championship defeat to Bristol City.
In Spain, Florian Lejeune played the full game as Alavés suffered a 1-0 defeat at home to Osasuna, while forward Yoshinori Muto made a substitute appearance for the final 20 minutes of Eibar’s 1-1 draw at Huesca.
And in Holland, Elias Sørensen made a late substitute appearance for Almere City on Sunday night.
The Danish forward came on for the final seven minutes of their home game against Cambuur-Leeuwarden, but couldn’t stop his side from slipping to a 2-0 defeat.