The 32-year-old didn’t feature as United lost 2-1 to Chelsea on Sunday, the Blues replacing them at the top of the Women’s Super League table
Heath was named on the bench as United travelled to Kingsmeadow but was stood at the side of the pitch with the likes of Alessia Russo and Lucy Staniforth, both unavailable due to injuries, while the team warmed up before the game.
The American sat among the substitutes for the match but was unused as United lost to the reigning Women’s Super League champions.
Heath had chosen to stay with her club rather than travelling to the United States for a training camp and two friendlies with the USWNT, who play Colombia on January 18 and January 22.
“Tobin got injured in the week and we wanted to give her as much time as possible to try and be involved,” Stoney said.
“Unfortunately, she woke up this morning and she wasn’t right, so it wasn’t right to play her.”
United went into the weekend leading the WSL standings, but slip to second on goal difference due to Sunday’s result.
Chelsea took the lead after half an hour through Pernille Harder, who squeezed the ball in from a corner after Fran Kirby had hit the post.
Lauren James equalised for United after coming off the bench, but Kirby decided the game just after the hour mark when she latched onto a long ball forward from goalkeeper Ann-Katrin Berger and finished clinically.
“If we’re going to press high, we have to be able to defend a long ball,” Stoney said, reflecting on that goal.
“For me, a goalkeeper should never get an assist. That tells you that you haven’t defended the situation very well. We have to head it, we have to clear it, we have to deal with it – we don’t.
“I’m not going to single anyone out because I think my defence, especially my centre-backs, have been outstanding in the first half of the season, but you have to stay switched on all the time.”
United were boosted by the return of Christen Press on Sunday, the forward having been suffering with illness for the last few weeks.
Press played 55 minutes in the game before being replaced by goalscorer James as she continues to build up her match fitness.