He amassed Primeira Liga totals of 282 matches and 33 goals over 12 seasons, representing in the competition Alverca, Gil Vicente, Vitória de Guimarães (three spells) and Benfica.

[7] He netted twice from 28 Cypriot First Division appearances in his debut campaign, but had the intention to leave after his contract expired due to financial differences, eventually changing his mind and also becoming captain.

On 14 July 2006, the Justice Council of the Portuguese Football Federation threw out the sentence on technicalities, specifically the disregard for the defence of Assis,[11] and removed the six-month suspension to the player after he was initially suspended for five months.

Benfica threatened to press charges to try and identify who was responsible for the false charges and for the whole procedure, and asked for the destitution of the laboratory director and the technicians involved in this case; on the 19th, Portuguese sports newspapers O Jogo and A Bola quoted the president of the laboratory that handled the sample and made the analysis (LAD, Anti-Doping Lab) claiming that the player had been tested positive for 19-norandrosterona, a steroid – according to the president statement, sample A contained 4.5 nanograms per milliliter (ng/ml) and the counter-sample 4.0 ng/ml.

The following day Vieira replied, accusing Luís Horta (president of LAD) and Luís Sardinha (president of the National Anti-Doping Council, CNAD) of fabricating data and lying to protect themselves – he brought to light that the meeting of CNAD that decided to prosecute the athlete did so disregarding the technical analysis that proposed that charges should be dropped, according to the meeting's agenda.

This appeal to the Court of Arbitration for Sport Archived 9 February 2014 at the Wayback Machine agreed with the agency and found that foul-play was with the player and the original six-month sentence was increased to one year.

[15] Assis returned to the national team after more than six years of absence,[16] taking the pitch during the second half of the decisive 2010 FIFA World Cup qualifier against Malta (4–0, in Guimarães).