The Ivorian faces a ban after the club said on Thursday that he had tested positive for a "specified substance" in an A-sample. He has yet to decide whether to request that the B-sample is tested.
The Football Association, club and UK Anti-Doping agency have not given a time frame for the legal process to be completed but coach Roberto Mancini must manage without his former captain for the immediate future at least.
While Joleon Lescott and Jerome Boateng coped well in the heart of defence in the midweek 3-0 FA Cup win over Aston Villa, they do not have the experience of Toure which could be vital in their pursuit of the FA Cup, Premier League and Europa League.
Mancini complained last week that he had "only 15 fit players" to cope with a cluttered fixture list and that his players were not "machines", suggesting tiredness would hamper the world's richest club's bid for a first trophy since 1976.
He will hope that Toure's usual central defensive partner Vincent Kompany recovers from a bruised hip in time for Saturday's home league game against Wigan Athletic to ease the impact of the Ivorian's absence even if there is no way of avoiding the inevitable media glare.
The length of Toure's suspension remains uncertain and precedents suggest he could face anything from a small punishment to a lengthy ban if the positive test is confirmed. The World Anti-Doping Agency (WADA) code stipulates that an athlete can be banned for a maximum of four years.
WADA also says a "specified substance" suggests there could be a "credible non-doping explanation" for its presence. Such an explanation could, for example, be taking a flu remedy containing a banned substance.
SPECIFIED SUBSTANCES
WADA adds that specified substances are not necessarily less serious agents for the purposes of sports doping than other prohibited substances.
"For that reason an athlete who does not meet the criteria under this article would receive a two-year period of ineligibility and could receive up to a four-year period of ineligibility under article 10.6 [relating to aggravating circumstances such as deception]," the WADA code says.
If Toure can prove the substance was present for reasons other than to improve performance, any ban is likely to be considerably shorter and he could even be given only a warning.
Former Sheffield United goalkeeper Paddy Kenny served a nine-month ban after testing positive for a banned stimulant in 2009 after proving it had been in a cough medicine he had taken to treat an infection rather than improve performance.
Earlier his year, Hamilton Academical midfielder Simon Mensing served a four-week ban after testing positive for a banned substance found in a dietary supplement.
Toure's peers were stunned by the failed test, particularly as he is known to be a devout Muslim who does not drink alcohol.
"Just read Kolo Toure failed a drugs test .... Jeeeez, I didn't see that coming," Manchester United defender Rio Ferdinand, who served an eight-month ban for missing a doping test in 2003, wrote on Twitter. "Gonna read up to fi
