In 2:105, God addresses the believers (per the preceding verse 2:104) and tells them that those who do not believe (including people of the book, explicitly mentioned in the verse) don't like that good be sent down to the believers from God. A plausible interpretation of this would be that the good that God is sending down to the believers is the Quran, and that the disbelievers don't like it out of jealousy which is hinted at as the verse goes on to say that God chooses whoever He wants. This interpretation is straightforward and found in the standard books of interpreting the Quran. Now, on to 2:106.
In 2:106, God mentions that if He abrogates a verse or causes it to be forgotten, He will bring better or similar to it. Given that the preceding verse is describing how the disbelievers don't like that the Quran was sent down to us, and the Quran implicitly renders the previous books not valid any more (which is probably the reason the people of those books don't like it), it is completely reasonable to infer that what is abrogated are the verses of the previous books such as the Bible, and that the Quran is what is better or similar which God brought. This interpretation also fits the "forgotten" part, since many original verses of the Bible and previous holy books were gone.
Al-Jabri postulates and elaborates the same interpretation on page 147 of
his book. He quotes Hijazi Al-Saqqa in his elaboration, saying that the biggest reason for the Jews to resist Islam at the time of the Prophet (PBUH) is that they liked the monopoly of revelation to stay in the Isaac branch of the Abrahamic heritage, and did not want the Ismail branch to receive such a blessing, and they contended that there can be no juristic revelation other than the Torah that they have since it is sufficient. This fits the wording of 2:105-106 perfectly. Al-Jabri also mentions that the forgotten revelations are those like the scripts of Abraham that were gone with time.
There is one linguistic point that convinces me that this is the correct interpretation of 2:106. The Arabic word " خير " is used in both 2:105 and 2:106. The word means both "good" and "better" and also "choice" as an adjective (describing something chosen for quality). In 2:105, "خير" is what the disbelievers don't like us to get from God, and in 2:106, "خير" describes what God promises to bring if He abrogates a verse. Coming in two adjacent verses, I believe that "خير" is referring to the same thing in both verses which would be the Quran, and this supports the interpretation that 2:106 is about the Quran abrogating previous books.
Even this point that I am so proud of
is covered by Al-Jabri at the top of page 151.
Of course it is possible that 2:106 also describes verses that had been gone from the Quran during the life of the Prophet (PBUH).
That's where Al-Jabri disagrees, as he believes no abrogation of any kind has taken place within Islam. However, Al-Jabri makes a conclusion on page 152 that is not overreaching. He concludes that 2:106 cannot be taken as evidence that there is abrogation in the Quran and stops at that.
IMHO, I don't see a fundamental problem with the possibility of abrogated/substituted/forgotten verses within Islam, that were gone and never made it to the text of the Quran, since it is the same as abrogating and forgetting previous revelations except on a smaller time scale. A better argument is the static/dynamic dichotomy, where the fact that the Quran was revealed over time may necessitate deletions along with additions. What matters is that the book that the Prophet (PBUH) left us is intact.