The Spaniard underwent surgery after rupturing a knee ligament in November and hoped to return to competitive action in March.
A problem with his Achilles, however, is delaying his recovery programme and has put his involvement in the Gunners' run-in in the Premier League title race in doubt.
"Santi is not doing too well," Arsene Wenger revealed. "He has some Achilles problems that hold him [back]. His knee is doing well but his Achilles has inflamed a little bit and we have to put him off impact.
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"It is a reaction to him coming back. We have to see now how he responds. He had always a little tendonitis on his Achilles and we have to see how he responds to that."
Jack Wilshere has not featured at all for Arsenal in 2015-16 after breaking his leg in pre-season and Wenger is likely to be without Alex Oxlade-Chamberlain for two months after he sustained a knee injury against Barcelona.
"Jack is doing quite well but has still not been outside yet," the manager added. "Running, I think, starts again this week. It will be three to four weeks [from there].
"Alex will be out for six for eight weeks. No surgery. We feared surgery at some stage before we saw the MRI."