McManus ended a four-and-a-half month injury lay off when he replaced Julio Arca in the 84th minute of Saturday's success, and calmly slipped between centre-halves Rhys Williams and Jonathan Grounds to help keep the Hornets out.
The Scotland international's experience has been sorely missed since the turn of the year, with injuries to both Matthew Bates and Andrew Davies leaving Boro short of central defensive cover.
McManus should be competing for a starting spot by the time Leicester visit the Riverside on April 2, and Mowbray expects the 29-year-old to be a key performer in the remaining nine matches.
"Stephen coming back for the last ten minutes was very good news for the squad," said the Boro boss. "He's got his boots back on and played football, and I'm pretty sure we'll need him through the month of April so it's good to have him back.
"He's a Scottish international who's captained Celtic in front of 60,000 every week, so games like this are no problem for him.
"He'd only had a few minutes for the reserves (before Saturday), but that was never a concern for me.
"We've conceded a few 94th-minute goals, so I thought putting him in the middle of two young players was a good idea. Seeing the game out was what was required."
With no game this weekend because of the international break, Mowbray is hoping to have more injured players back in contention by the time Leicester travel north in 11 days time.
Seb Hines should be available after sitting out Saturday's victory, while Willo Flood and Justin Hoyte could also be back in full training before the start of next week.
"At the moment, it feels as though we're having to play players with injuries," said Mowbray. "There are a few players on the treatment table, but hopefully there'll be a few back by the time of the next game because April is a big month for us.
"We've got games just about every midweek, and we've got players who won't be able to play in all those games. We'll have to utilise the squad as best we can."
* Middlesbrough's matchday programme, REDSQUARE, won the CSI Sports Best Matchday Programme award at the Football League Awards in London on Sunday night.
Boro's programme, which features a collection of photographs compiled from The Northern Echo archives, beat off competition from Burnley, Cheltenham, Huddersfield, Rotherham and Swindon.
Source: Northern Echo
Source: Northern Echo