The following is an excerpt from Dr. Thomas Holland's Crowned With Glory, ©2000, used with permission.
Mark 1:2 - "As it is written in the prophets"
The Traditional Text reads, "As it is written in the prophets," and then cites from Malachi 3:1 and Isaiah 40:3. Other texts read, "As it is written in the Prophet Isaiah," before quoting Malachi and Isaiah. The reading of the Traditional Text has considerable support. It is found in many of the Greek uncials (A, K, P, W, P) the majority of Greek minuscules (28, 1009, 1010, 1079, 1195, 1216, 1230, 1242, 1252, 1344, 1365, 1546, 1646, 2148) and the majority of Greek lectionaries. Thus the Greek support dates from the fourth century onward. Additionally we also find the same reading in the Syriac Harclean version (616 AD), the Armenian version (fourth/fifth century) and the Ethiopic versions of the sixth century. It also receives patristic citations from many of the church fathers such as the Latin version of Irenaeus (202 AD), Photius (895 AD), and Theophylact (1077 AD).
Contextually there arises a problem with the reading as found in the Critical Text. The passage cites both the Prophet Malachi (3:1) and the Prophet Isaiah (40:3). The reading, "As it is written in Isaiah the Prophet," seems inconsistent. Nevertheless, it has been noted that Isaiah was the major prophet and therefore he takes preeminence over Malachi. To illustrate this point, scholars often refer to Matthew 27:9. This passage, so is claimed, is not really a citation of Jeremiah but instead comes from Zechariah 11:12. Jeremiah receives the preeminence as the major prophet.
However, this point can be argued. The text in Matthew does not say it was written as the passage in Mark does. Instead, the text in Matthew states, "Then was fulfilled that which was spoken by Jeremy." God, the Author of Scripture, is aware of who writes what and who speaks what. Simply because Zechariah writes the passage does not mean Jeremiah did not speak it. Also, Zechariah warned Israel to pay attention to what the former prophets had spoken (Zechariah 7:7). The ancient Jews had a saying that, "the spirit of Jeremiah was in Zechariah."  Much of what Zechariah received, he did so from both the Lord and the former prophet, Jeremiah.
The position presented by many that some copyist made the change from "Isaiah the Prophet" to "the prophets" in Mark 1:2 in order to correct what was perceived as a possible error is conjecture.  One can just as easily speculate that an Egyptian copyist not overly familiar with Jewish Old Testament prophets recognized the Isaiah quote and made the change for what he considered to be better clarity. The point still remains that both sides have textual support for their respective positions. It also is understood, as Dr. George Kilpatrick has noted, that most of these types of textual variants were introduced into the manuscripts by the second century.  Therefore, one reading is as likely (textually speaking) as the other. The difference is contextually. It is more truthful to say "the prophets" when citing two prophets. Accordingly, the reading in the Traditional Text is both textually substantial and contextually correct.
 In his Bible commentary regarding Matthew 27:9, Matthew Henry notes this saying of the Jews concerning the spirit of Jeremiah being in Zechariah.
 Bruce Metzger, A Textual Commentary On The Greek New Testament (New York: United Bible Societies, second ed., 1994), 62.
 George D. Kilpatrick, The Principles And Practice Of New Testament Textual Criticism, (Belgium: Leuven University Press, 1990), 34.