The Red Devils winger started his career alongside Stewart at Watford before the England international sealed a move Aston Villa in 2007.
Young was part of the United side that were 3-1 winners against Stewart’s current employers, San Jose Earthquakes, at Avaya Stadium earlier this week.
And the 30-year-old uploaded a snap with his former Hornets team-mates after he caught up with the Major League Soccer star in San Jose.
Young wrote on Instagram: “Always good to catch up with this character @jordan.stewart3 #SanJose #ToTheFace”
The former Villa winger enjoyed something of a renaissance under Louis van Gaal last season after he secured a regular spot at left wing-back.
Young, who was part of United’s title-winning side in 2013, scored two goals in 29 appearances in all competitions for the Red Devils.
The 20-time English champions face Barcelona and Paris Saint-Germain in their final games of the pre-season tour of America.
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