Toshack, who described last night's friendly victory as "just what the doctor ordered", said of Koumas: "He has had a dreadful 12 months, and it was good to see him back playing at this level.
"His weight of pass and shooting like that was a joy to watch and was as good a performance as he could have wished for."
Koumas is suspended for Wales' next Euro 2008 qualifier in Dublin against the Republic of Ireland in March.
"That is a shame, because he has become very important for us," said Toshack.
Koumas is a certainty to play in Wales' next friendly against Northern Ireland in Belfast in February, however - and will be back for the competitive match after Dublin, against San Marino.
Toshack added: "Jason has developed well and has got his form and fitness back. He has accepted the responsibility in central midfield and can only improve now with us."
The Wales boss also hailed Craig Bellamy's all-action display.
The Liverpool striker had a hand in two goals and scored himself for the second successive time for Wales, having had a miserable season so far with his club.
Toshack said: "Craig was outstanding.
"His pace always caused them trouble and he created half-a-dozen chances with his running and crossing, before Chris Llewellyn finally scored from one at the end.
"Craig's all-round game was excellent. He is an unselfish player and will run and chase all night.
"Now Craig has had three successive excellent games for us, I believe that will help him find his confidence when he goes back to his club.
"It was a comfortable win in the end, but we were a little concerned at the start with a second-string back line and a new goalkeeper.
"Opposition at this level never give in; they always work and run - and they gave us a few difficult moments."
Toshack revealed that Richard Duffy came off with a groin injury, while captain Ryan Giggs was replaced at the break as a precaution because of a bruised foot.
"Someone stood on Ryan's foot. It started to swell up at half-time, and we did not want to take any chances with him and start annoying people," Toshack reported.
"I doubt it will cause him any trouble when he gets back to Manchester United.