When a Muslim asks me: what do you answer to the claim that the Injeel is corrupted, I answer with
Firstly: Define corruption? To what extent do you mean corrupted? Are you referring to local corruption or universal? Are you referring to the term 'corruption' often utilized within the field of Biblical criticism, which is slightly different from the typical muslim understanding of the word?
Secondly: Prove from primary sources that the modern muslim view of Bible corruption took place:
I want to know 1) who corrupted it, their names 2) where it was corrupted, the city and country, 3) when it was corrupted, which year, which century, 4) how it was corrupted, 5) why it was corrupted.
I would ask the muslim to provide primary sources for all these three.
Now the Muslim might refer to textual criticism and claim that corruption took place gradually. However, the corruption referred to in textual criticism is a completely different matter than the corruption pre-supposed by modern muslims.
Within textual criticism there is doubt about the 5 procent of the NT, this is not to say that remaining is corrupt, but rather that we do not have sufficient lacks the earliest attestation and cannot be effectively composed by the manuscripts, unfortunately for the muslims this does not imply that meaning of these passages are corrupted or missing. Hence this remaining part is left for our own individual belief or disbelief. Thus it would still confirm that Jesus died and resurrected, it would leave within the gospel the narrative and the sayings; this would hardly fit the nature of the Book Allah gave to Jesus.
Now the Muslim may go beyond textual criticism to source criticism, however now were are gona have a number of problems; firstly, this approach is utterly speculative, yet if factual as many muslims claim, it would would exclude Jesus' miracles, virgin birth (which are found in the Qur'an), it would exclude the paraclete (the presupposed Muhammad); it would leave utterences related to Jesus being the Son of God and Jesus being the one to baptise in the Holy Spirit as intact, which both contradict the Qur'an.
Hence if the modern muslim theory of corruption took place, the muslim needs to consider the five questions I asked earlier.
Thirdly: if the Injeel is corrupt, how will the Muslim respond to his own book, the Qur'an, which does not state that the Injeel was corrupted, but rather attests to its preservation and accuracy Muhammad's era.