Why Did Shaykh Mufid have Mutazilli Teachers ?

The 12rs say that the 12th imam is the imam of the time, but after reading the biography of Shaykh Mufid it states that he had Mutazilli teachers. I was refuted on shi'achat. However, at that time I was relying on a orientalist source. The other day I came across a mainstream Shia site which states that this Shaykh did study under the Mutazilli.

He was born in the year 338 A.H./949 and was brought up in a village. His father brought him to Baghdad for his education. There he studied under Shi'i and Mu'tazili scholars. He showed such promise that one of his teachers recommended that he study under one of the leading scholars of the period, 'Ali b. 'Isa al-Ramani. He also studied under the leading Shi'i traditionists of the time, al-Shaikh al-Saduq.
http://www.al-islam.org/al-serat/irshad-howard.htm

Ali Ibn-e-Isa ar-Ramani is a Mutazilli scholar.