Watanabe was promoted from his role as assistant to Graham Arnold on Wednesday after the Australian left the club by mutual consent just five months into his tenure.
Arnold was winless in his six league games at the helm, but his successor made an instant impact on Saturday as Shingo Akamine scored both goals in the second half at the Nissan Stadium.
The forward had not found the back of the net before facing Yasuhiro Higuchi's side, but struck 66 minutes in and then sealed the points 10 minutes from time.
Victory for Sendai still leaves them second-bottom of the table, but they will now be hoping to kick on under 40-year-old Watanabe.
It was also a good day for champions Sanfrecce Hiroshima on Saturday, as they returned to the top of the table with a 1-0 home win over Tokyo.
Kazuhiko Chiba's first goal of the season 10 minutes from time sealed victory and ensured Sanfrecce replaced Kashima Antlers at the summit after the latter were beaten 2-1 at home by Albirex Niigata.
The Antlers are now a point behind Hajime Moriyasu's men, with Takeshi Aoki's own goal 17 minutes from time proving to be costly afterShoma Doi had cancelled out Kemgo Kawamata's opener for the visitors - who are up to sixth spot.
Brazilian duo Fabio Simplicio and Marquinhos were on target as Vissel Kobe beat Tokushima Vortis 3-0 to move up to third place and leave their opponents still searching for a first point.
Cerezo Osaka forward Diego Forlan scored twice but Gamba Osaka midfielder Hiroyuki Abe grabbed a brace of his own to ensure Saturday's derby at Nagai Stadium finished 2-2.
Urawa Reds moved up to fourth by winning 2-1 at Nagoya Grampus, while Sagan Tosu won 2-0 at home to Ventforet Kofu and Shimizu S-Pulse beat Omiya Ardija by the same scoreline.
Kawasaki Frontale drew 1-1 with Kashiwa Reysol in the only game to be played on Friday.
