The Red Devils came from a goal down to triumph 3-1 at Braga in the UEFA Champions League on Wednesday night, with Robin van Persie, Wayne Rooney and Javier Hernandez all getting on the scoresheet.
The result saw them through to the knockout rounds as winners of Group H and Phelan believes it was important to respond after exiting at the group stage last term.
Asked how crucial qualification was, he told Sky Sports: "Very important. We had a disappointing spell in Europe last year and we wanted to put that right as quickly as possible.
"I think the players have done that tonight and now we can look forward to league football for a while and kick on in the Champions League when it matters."
Phelan admits United made it tough for themselves after going a goal behind to an Alan penalty in Portugal.
He added: "I think what we have discovered through all our games in Europe is that you can't take anything for granted.
"I thought it was a difficult game, or we made it a difficult game tonight.
"We're thankful that we've got the points and now we're top of the group."