England pair Jack Nowell and George Kruis were both injured on Saturday, but Manu Tuilagi returned to action after well over a year out injured.
Exeter wing Nowell twisted his knee against Gloucester, while Saracens lock Kruis was concussed against Harlequins and taken off on a stretcher.
But, in front of new England boss Eddie Jones, Tuilagi came off the bench for Leicester as they beat Northampton.
Saints hooker Dylan Hartley missed the game because of a "bang to the ribs".
England's Six Nations campaign begins against Scotland on 6 February and Jones is due to reveal his first England elite squad squad since taking over from Stuart Lancaster on Wednesday.
Nowell injured his knee in the 19-10 victory over the luckless May's club, Gloucester, although the extent of the injury is not yet known.
Exeter backs coach Ali Hepher said Nowell had picked up a "twisted knee" and would be "assessed in the week", but added: "Sometimes the least painful ones can be the longest out."
Kruis was struck by the swinging arm of James Horwill just 90 seconds into the 29-23 defeat by Harlequins at the Twickenham Stoop and was tended to by medics for eight minutes before being carried off on a stretcher.
Saracens director of rugby Mark McCall said: "George is fine. There is nothing wrong other than the concussion. He was knocked out. He'll do the normal checks."
He will now be assessed ahead of Saturday's Champions Cup match with Ulster.
Tuilagi played his first game since suffering a serious groin injury in October 2014.
The 24-year-old managed 20 minutes after coming on in the second half of the thrilling 30-27 win against Northampton.
He was a late inclusion on the Leicester bench, with his appearance in the match-day squad only announced less than an hour before kick-off as Leicester looked to dampen down excitement ahead of his return.
And Leicester boss Richard Cockerill urged caution over the block-busting centre's fitness for the Six Nations opener at Murrayfield.
He said: "I will speak to Eddie [Jones] in the next few days. Manu will keep improving, but the start of the Six Nations is unrealistic because we need to make sure he is 100% right."
Northampton hooker Hartley, a contender for the England captaincy, was forced to withdraw from the Saints replacements shortly before the game with a rib injury.
Northampton boss Jim Mallinder said: "He got a bang to his ribs last week. He trained a little bit in the week but at the last minute, he was just too sore to play today. He is not going to be out for a long time."