Lining up first in the tucked-in, left-sided position, the Japanese may not have made much of an impact on the stats sheet, but his contribution towards the team's fluidity and poise was indisputable.
At times it seemed as though Kagawa was the pivot around which the rest of the team revolved.
Shifting their balance as a unit, United attacked in a whirling, shape-shifting pattern reminiscent of the football played during the years in which Rooney, Tevez and Ronaldo switched positions in place of a recognised striker.
Over the past few seasons, the club have reverted to a more positionally-strict approach, attacking predominantly down the right side. This is partly due to the potency of Valencia and Rafael's partnership on that flank with the left being manned by a wide-lying goal threat, such as Ashley Young, who replaced Kagawa on the 70th minute.
Having won two Premier League titles with such a rigid setup, there's no doubt about its effectiveness within the domestic game.
But in Europe, the club have perhaps been lacking that special something to multiply the squad's undoubted qualities.
Against Leverkusen, Kagawa looked to be the perfect catalyst to bring back such a fluent yet composed style of play. With the Japanese occupying the left channel, moving up and down the space to receive and offer the ball, United's other forwards spun and weaved around him, overloading the left wing to target the opposing right-back, Giulio Donati.
If the threat posed by United's right wing is all about the power and pace of Valencia and Rafael, with Kagawa in place the left flank appears to be a credible and complementary counterpoint based on finesse, picking at Leverkusen's weak point through superior technique and movement.
Though his moments on the ball may have been limited, the confidence he gave those around him, knowing that the Japanese could be relied upon to control and retain the ball if required, was telling.
Van Persie and Rooney swapped positions and roamed even more than normal due to Kagawa's teasing advances forward and back, pulling defenders away and peeling into space.
At different points the Dutchman alternatively stepped back to play as a No. 10 or drifted extremely wide to the left, playing almost like a winger and offering service to the box not unlike his dead-ball delivery.
Rooney too wandered out to the right far more than usual, with Kagawa ever-present, tucked in on the other side and allowing the Englishman to head into other areas requiring his attention.
This freeing up of his duties to slide into the left side as usual helped Rooney make himself more available to take advantage of goal-scoring opportunities.
With van Persie dropping back and sliding left, and Rooney enjoying even more free reign to contribute where he can, Kagawa was at times the highest-lying United player, a position that allowed him once again to preoccupy defenders as his team-mates rushed into the box proper.
Much has been made of the supposedly-irresistible combination of United's two headline-makers, but last year they failed consistently to click due to injuries and confusion over status.
On the evidence of his first and long overdue appearance of the season, Kagawa could be the facilitator to ease both strikers' understanding.
David Moyes' formation may well be a lopsided 4-4-2 on paper, but in motion it performs more like a 4-3-3 and maybe something rather less describable in numbers once Patrice Evra is bombing on and overlapping the tucked-in, left-of-the-hole Kagawa.
Perhaps this is exactly how Moyes has always seen his preferred system working with the right personnel. The way United played against Leverkusen wasn't too far removed from Everton at their best under the Scotsman, although with a higher level of invention and quality.
If Kagawa's deployment in Moyes' first Champions League game as United manager has any bearing on how his new manager will use him under his tenure, then his inclusion, now match fit, has been worth the wait.
The signing of Marouane Fellaini—to shore up the midfield as a potential perfect partner to Michael Carrick—is the foundation to the interplay that is now possible in front of them yet notice how United returned to a less unpredictable approach once Kagawa was substituted for Young.
Forget the stats, the goalscorers and the assists, and instead appreciate the more intangible yet clear influence his abilities bring.
Finally freed, Kagawa has returned the European savvy and class to United in a manner that will have delighted Moyes and ensured him of many more games this season.
And to think, this is how the attacking midfielder plays while lacking match practice.
Once Kagawa is up to speed and in the team full-time, Rooney and van Persie will be able to lose themselves in their football week in, week out.
Ignore the numbers and see the reality. Kagawa could be United's title winner and key to a return to Europe's elite.