The 29-year-old was quick to voice his disappointment at suffering elimination on penalties at the hands of APOEL Nicosia, but stressed that Lyon will be back on form
Les Gones, who were leading 1-0 from the first leg at Stade Gerland, fell behind in the ninth minute when Gustavo Manduca fired in from close range after some poor defending to nudge Ivan Jovanovic's men ahead on the night and level on aggregate in their last-16 tie.
Although the visitors, including Kallstrom, had chances throughout normal time and extra-time, the tie would eventually be decided on penalties.
With the shootout finely poised at 3-3 after three rounds, Alexandre Lacazette and Michel Bastos had their decisive penalties saved by Dionisis Chiotis as APOEL's fairytale run in Europe's premier club competition continued into a maiden quarter-final appearance.
And the 29-year-old was left disappointed at the manner of his side's elimination.
"It is the worst way to go out. Over the two games, I think we should have scored more at home [in the first leg]. Today, it was much more balanced." the Sweden told Uefa.com.
"And when it comes to the penalty shootout anything can happen. We're in a tough situation now. But we'll fight to be back."
Remi Garde's men, eliminated at this stage for the second time in two seasons, now face a tough fixture list with games against Lille this Saturday, to be followed by the Rhone derby against Saint-Etienne and the Coupe de France tie against PSG as les Gones now seek to improve their seventh-place standing in Ligue 1.
Ivan Jovanovic's men, meanwhile, can dare to dream as they prepare for the quarter-finals which begin at the end of the month. They host mid-table Enosis in the Cypriot First Division on Monday.