Prior to working with Armstrong, Cullen was physio to Lewis Hamilton, joining his camp in 2016 and accompanying the seven-time world champion throughout his title winning years at Mercedes.
Cullen now trains IndyCar star Marcus Armstrong who races for Chip Ganassi in the series.
Since joining his team, Cullen has had a profound impact on the career of her compatriot.
The weekend Cullen came on board, Armstrong earned a season-best fifth at the Thermal $1 Million Challenge, a result later replicated at the Grand Prix of Indianapolis.
However, the Kiwi suffered an engine failure during the first caution period, forcing him to retire only six laps in - without completing a single lap in live racing.
Doubtless Armstrong and Cullen will be heartbroken with the result, but will be determined to improve for their next outing in Detroit.