This is a media cliche that simply isn't true.
CL quarterfinalists:
Barca, Real, Valencia.
3 of the top 5 in Spain along with Deportivo and Zaragosa.
Bayern - 2nd in Germany
Lazio - won Italien title
Porto, 2nd in Portugal
Chelsea - 5th after 3rd place finish last year, Chelsea are not 5th just because of the CL - you could argue that Man U, Arsenal, Leeds and Liverpool are simply better teams.
Man U - won league with massive 18 point margin.
Don't forget Sporting's problems with Materazzi and the turmoil the club was in until November.
I think this had a more detrimental effect on Sporting than the CL on Porto.